Don't print the "googleimage.lua: didn't return a string" message if the function returned nil.

modules/misc/lua/luameta.c

@@ -178,7 +178,7 @@ static int fetch_art( vlc_object_t *p_this, const char * psz_filename,
                 i_ret = VLC_SUCCESS;
             }
         }
-        else
+        else if( !lua_isnil( p_state, s ) )
         {
             msg_Err( p_this, "Lua art fetcher script %s: "
                  "didn't return a string", psz_filename );