Update libxml test for the directory field behaviour change

See https://gitlab.gnome.org/GNOME/libxml2/-/issues/753.
The base directory for the entity is no longer set, follow the upstream
behaviour.

ext/libxml/tests/libxml_set_external_entity_loader_variation1.phpt

@@ -61,7 +61,7 @@ string(13) "-//FOO/ENTITY"
 string(32) "http://example.com/fooentity.ent"
 array(4) {
   ["directory"]=>
-  string(%d) "%s"
+  %r(NULL|string\(%d\) "%s")%r
   ["intSubName"]=>
   string(3) "foo"
   ["extSubURI"]=>