cli: handle empty/unset values for "gsm.apn" property

Most string properties can be either %NULL (unset) or a non-empty
string.

For a few properties, like "gsm.apn", also the empty word is a valid
value. That makes it problematic to use from nmcli, because

  nmcli connection modify "$PROFILE" gsm.apn ""

means to reset the default (NULL). How to configure the empty word?
For the APN, "" has a specific meaning, distinct from NULL, so we
need to be able to represent that.

The other problem with nmcli is that

  nmcli -g gsm.apn connection show "$PROFILE"

is supposed to give you a value that you an set again, like

  X="$(nmcli -g gsm.apn connection show "$PROFILE"; echo x)"
  nmcli connection modify "$PROFILE2" gsm.apn "${X%$'\n'x}"

but for %NULL and "" the output would be the same.

The "solution" to that is interpreting "" as NULL (like we always did)
and a non-empty string that contains all whitespace, like a string with
one whitespace less. This way, all values can be expressed.

Note that in case of "gsm.apn", the string is anyway internally
normalized with g_strstrip(), so a string with all whitespace was
not expressable.

src/libnmc-setting/nm-meta-setting-desc.c

@@ -5692,6 +5692,9 @@ static const NMMetaPropertyInfo *const property_infos_GSM[] = {
         .property_alias =               "apn",
         .prompt =                       N_("APN"),
         .property_type =                &_pt_gobject_string,
+        .property_typ_data = DEFINE_PROPERTY_TYP_DATA_SUBTYPE (gobject_string,
+            .handle_emptyunset =        TRUE,
+        ),
     ),
     PROPERTY_INFO_WITH_DESC (NM_SETTING_GSM_NETWORK_ID,
         .property_type =                &_pt_gobject_string,

src/tests/client/test-client.check-on-disk/test_003.expected

@@ -182,12 +182,12 @@ id
 path
 uuid
 <<<
-size: 4536
+size: 4538
 location: src/tests/client/test-client.py:test_003()/14
 cmd: $NMCLI con s con-gsm1
 lang: C
 returncode: 0
-stdout: 4403 bytes
+stdout: 4405 bytes
 >>>
 connection.id:                          con-gsm1
 connection.uuid:                        UUID-con-gsm1-REPLACED-REPLACED-REPL
@@ -273,7 +273,7 @@ gsm.number:                             --
 gsm.username:                           --
 gsm.password:                           <hidden>
 gsm.password-flags:                     0 (none)
-gsm.apn:                                xyz.con-gsm1
+gsm.apn:                                "xyz.con-gsm1"
 gsm.network-id:                         --
 gsm.pin:                                <hidden>
 gsm.pin-flags:                          0 (none)
@@ -288,12 +288,12 @@ proxy.pac-url:                          --
 proxy.pac-script:                       --
 
 <<<
-size: 4566
+size: 4568
 location: src/tests/client/test-client.py:test_003()/15
 cmd: $NMCLI con s con-gsm1
 lang: pl_PL.UTF-8
 returncode: 0
-stdout: 4423 bytes
+stdout: 4425 bytes
 >>>
 connection.id:                          con-gsm1
 connection.uuid:                        UUID-con-gsm1-REPLACED-REPLACED-REPL
@@ -379,7 +379,7 @@ gsm.number:                             --
 gsm.username:                           --
 gsm.password:                           <hidden>
 gsm.password-flags:                     0 (brak)
-gsm.apn:                                xyz.con-gsm1
+gsm.apn:                                "xyz.con-gsm1"
 gsm.network-id:                         --
 gsm.pin:                                <hidden>
 gsm.pin-flags:                          0 (brak)
@@ -757,7 +757,7 @@ gsm.number:                             --
 gsm.username:                           --
 gsm.password:                           <hidden>
 gsm.password-flags:                     0 (none)
-gsm.apn:                                --
+gsm.apn:                                ""
 gsm.network-id:                         --
 gsm.pin:                                <hidden>
 gsm.pin-flags:                          0 (none)
@@ -863,7 +863,7 @@ gsm.number:                             --
 gsm.username:                           --
 gsm.password:                           <hidden>
 gsm.password-flags:                     0 (brak)
-gsm.apn:                                --
+gsm.apn:                                ""
 gsm.network-id:                         --
 gsm.pin:                                <hidden>
 gsm.pin-flags:                          0 (brak)
@@ -878,33 +878,33 @@ proxy.pac-url:                          --
 proxy.pac-script:                       --
 
 <<<
-size: 465
+size: 466
 location: src/tests/client/test-client.py:test_003()/24
 cmd: $NMCLI -g all con s con-gsm3
 lang: C
 returncode: 0
-stdout: 326 bytes
+stdout: 327 bytes
 >>>
 connection:con-gsm3:UUID-con-gsm3-REPLACED-REPLACED-REPL::gsm::no:0:-1:0:-1:0:no:::::-1::0:unknown:default:-1:-1:-1
 ipv4:auto::: :0::::-1:0::no:no:::0:yes:::0x0:no:yes:-1::
 ipv6:auto::::0::::-1:0::no:no:no:yes:-1:stable-privacy:0:::0:yes::0x0:
 serial:5:8:even:1:100
-gsm:no:::<hidden>:0:::<hidden>:0:no::::auto
+gsm:no:::<hidden>:0: ::<hidden>:0:no::::auto
 proxy:none:no::
 
 <<<
-size: 475
+size: 476
 location: src/tests/client/test-client.py:test_003()/25
 cmd: $NMCLI -g all con s con-gsm3
 lang: pl_PL.UTF-8
 returncode: 0
-stdout: 326 bytes
+stdout: 327 bytes
 >>>
 connection:con-gsm3:UUID-con-gsm3-REPLACED-REPLACED-REPL::gsm::no:0:-1:0:-1:0:no:::::-1::0:unknown:default:-1:-1:-1
 ipv4:auto::: :0::::-1:0::no:no:::0:yes:::0x0:no:yes:-1::
 ipv6:auto::::0::::-1:0::no:no:no:yes:-1:stable-privacy:0:::0:yes::0x0:
 serial:5:8:even:1:100
-gsm:no:::<hidden>:0:::<hidden>:0:no::::auto
+gsm:no:::<hidden>:0: ::<hidden>:0:no::::auto
 proxy:none:no::
 
 <<<