libgimpconfig: serialize empty value arrays as (property-name 0)

not (property-name NULL), which means nothing to the parser.
A "0" simply means an array with zero values.

libgimpconfig/gimpconfig-serialize.c

@@ -479,7 +479,7 @@ gimp_config_serialize_value (const GValue *value,
         }
       else
         {
-          g_string_append (str, "NULL");
+          g_string_append (str, "0");
         }
 
       return TRUE;