app: gimp_container_get_name_array() should always return a GStrv.

Even if the container is empty, then we return a GStrv of length 1 (i.e.
an array of length 1, terminating with NULL).

In particular, it improves gimp_container_get_filtered_name_array() as
well, and in turn various list-returning functions in libgimp. This
makes the API more consistent after changes from commit 8eb7f6df9e.

Note that a NULL return can be acceptable for error cases, but an empty
list because this is the expected result for the request should be an
empty GStrv, not NULL.

app/core/gimpcontainer.c

@@ -1000,13 +1000,11 @@ gimp_container_get_name_array (GimpContainer *container)
 {
   gchar **names;
   gchar **iter;
-  gint length;
+  gint    length;
 
   g_return_val_if_fail (GIMP_IS_CONTAINER (container), NULL);
 
   length = gimp_container_get_n_children (container);
-  if (length == 0)
-    return NULL;
 
   names = iter = g_new0 (gchar *, length + 1);