libnm-core: allow G_TYPE_STRV to be NULL

Otherwise we assert in _nm_utils_strv_to_slist() when setting a property,
using the code like:
g_value_init (&value, G_PARAM_SPEC_VALUE_TYPE (param_spec));
g_param_value_set_default (param_spec, &value);
g_object_set_property (G_OBJECT (setting), prop, &value);

e.g:
nmcli con mod my_profile eth.mac-address-blacklist "02:14:20:e6:16:83"

(changed by commit 6a4127cfa0)

libnm-core/nm-utils.c

@@ -681,9 +681,7 @@ _nm_utils_strv_to_slist (char **strv)
 	int i;
 	GSList *list = NULL;
 
-	g_return_val_if_fail (strv != NULL, NULL);
-
-	for (i = 0; strv[i]; i++)
+	for (i = 0; strv && strv[i]; i++)
 		list = g_slist_prepend (list, g_strdup (strv[i]));
 
 	return g_slist_reverse (list);
@@ -696,6 +694,9 @@ _nm_utils_slist_to_strv (GSList *slist)
 	char **strv;
 	int len, i = 0;
 
+	if (slist == NULL)
+		return NULL;
+
 	len = g_slist_length (slist);
 	strv = g_new (char *, len + 1);