cli: hide default setting of "connection.mud-url" from nmcli output

"connection.mud-url" is a commonly not used parameter, that most
users won't care. To minimize the output of

  $ nmcli connection show "$PROFILE"

hide the MUD URL if it is unset.

This mechanism of nmcli is not yet great, because there is currently
no way to print a default value, and

  $ nmcli -f connection.mud-url connection show "$PROFILE"

does not work as one would expect(??). But that is a shortcoming of the
general mechanism in nmcli, and not specific to the MUD URL property.

clients/common/nm-meta-setting-desc.c

@@ -5263,6 +5263,7 @@ static const NMMetaPropertyInfo *const property_infos_CONNECTION[] = {
 	),
 	PROPERTY_INFO_WITH_DESC (NM_SETTING_CONNECTION_MUD_URL,
 	    .property_type =                &_pt_gobject_string,
+	    .hide_if_default =              TRUE,
 	),
 	PROPERTY_INFO_WITH_DESC (NM_SETTING_CONNECTION_WAIT_DEVICE_TIMEOUT,
 	    .property_type =                &_pt_gobject_int,

clients/tests/test-client.check-on-disk/test_002.expected

@@ -502,12 +502,12 @@ NAME   UUID                                  TYPE      DEVICE
 con-1  5fcfd6d7-1e63-3332-8826-a7eda103792d  ethernet  --     
 
 <<<
-size: 1306
+size: 1263
 location: clients/tests/test-client.py:test_002()/23
 cmd: $NMCLI c s con-1
 lang: C
 returncode: 0
-stdout: 1181 bytes
+stdout: 1138 bytes
 >>>
 connection.id:                          con-1
 connection.uuid:                        5fcfd6d7-1e63-3332-8826-a7eda103792d
@@ -532,16 +532,15 @@ connection.metered:                     unknown
 connection.lldp:                        default
 connection.mdns:                        -1 (default)
 connection.llmnr:                       -1 (default)
-connection.mud-url:                     --
 connection.wait-device-timeout:         -1
 
 <<<
-size: 1318
+size: 1275
 location: clients/tests/test-client.py:test_002()/24
 cmd: $NMCLI c s con-1
 lang: pl_PL.UTF-8
 returncode: 0
-stdout: 1183 bytes
+stdout: 1140 bytes
 >>>
 connection.id:                          con-1
 connection.uuid:                        5fcfd6d7-1e63-3332-8826-a7eda103792d
@@ -566,7 +565,6 @@ connection.metered:                     nieznane
 connection.lldp:                        default
 connection.mdns:                        -1 (default)
 connection.llmnr:                       -1 (default)
-connection.mud-url:                     --
 connection.wait-device-timeout:         -1
 
 <<<