codec: ttml: don't validate namespace if it's NULL

Similar to d16c61873f.

Fixes #29501

modules/codec/ttml/substtml.c

@@ -742,7 +742,7 @@ static int ParseTTMLChunk( xml_reader_t *p_reader, tt_namespaces_t *p_nss,
                 break;
 
             case XML_READER_STARTELEM:
-                if( strcmp( psz_node_namespace, TT_NS ) ||
+                if( ( psz_node_namespace && strcmp( psz_node_namespace, TT_NS ) ) ||
                     strcmp( tt_LocalName( psz_node_name ), "tt" ) ||
                     *pp_rootnode != NULL )
                     return VLC_EGENERIC;