DOC: Fix missing cross-reference checks for sphinx-tags

In cda4372, I "simplified" from
`node['refdomain']` to the existing `domain` argument. With
`sphinx-tags`, the `warn-missing-reference` event occurs for a `None`
domain, and we crash trying to access `domain.name`. But
`node['refdomain']` is still an empty string, so revert back to that.

Author: QuLogic

doc/sphinxext/missing_references.py

@@ -100,10 +100,11 @@ def handle_missing_reference(app, domain, node):
     #. record missing references for saving/comparing with ignored list.
     #. prevent Sphinx from raising a warning on ignored references.
     """
-    typ = node["reftype"]
+    refdomain = node["refdomain"]
+    reftype = node["reftype"]
     target = node["reftarget"]
     location = get_location(node, app)
-    domain_type = f"{domain.name}:{typ}"
+    domain_type = f"{refdomain}:{reftype}"
 
     app.env.missing_references_events[(domain_type, target)].add(location)