keyfile: avoid assertion failure in nm_keyfile_plugin_kf_get_{string,integer}_list()

g_key_file_get_integer_list() can return %NULL without setting an error.
That is the case if the key is set to an empty value.

For X sake, this API. Read the documentation and figure out whether
the function can return %NULL without reporting an error.

Anyway, avoid the assertion failure.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/412
(cherry picked from commit 8f46425b11)
(cherry picked from commit 97139f5e3d)

libnm-core/nm-keyfile-utils.c

@@ -124,7 +124,6 @@ nm_keyfile_plugin_kf_get_##stype##_list (GKeyFile *kf, \
 			list = g_key_file_get_##stype##_list (kf, alias, key, &l, &local); \
 		} \
 	} \
-	nm_assert ((!local) != (!list)); \
 	if (local) \
 		g_propagate_error (error, local); \
 	if (!list) \