use proper initialization NULL for CORS variable, not an empty string (openstreetmap#479)

The empty string as initialization for CORS breaks the tile access in case Origin header is sent. Properly initialize it with the correct value NULL. Error was introduced 10 months ago it seems.

Author: stoecker

src/mod_tile.c

@@ -2010,7 +2010,7 @@ static const char *add_tile_config(cmd_parms *cmd, void *mconfig, int argc, char
 		return "AddTileConfig error, the configured zoom level lies outside of the range supported by this server";
 	}
 
-	return _add_tile_config(cmd, baseuri, name, minzoom, maxzoom, 1, 1, fileExtension, mimeType, "", "", "", "", tile_dir, 0);
+	return _add_tile_config(cmd, baseuri, name, minzoom, maxzoom, 1, 1, fileExtension, mimeType, "", "", "", NULL, tile_dir, 0);
 }
 
 static const char *load_tile_config(cmd_parms *cmd, void *mconfig, const char *config_file_name)