Fix sphinx-doc#9322: KeyError is raised on PropagateDescDomain transform

PropageteDescDomain applies the domain name from the "domain" attribute
of parent node (desc node) to the desc_signature node.  The structure
has longly generated by ObjectDescription.  But it must not be a new
rule.

This allows to build document that contains non standard doctree.

CHANGES

@@ -76,6 +76,7 @@ Bugs fixed
   whitespace (ex. ``Dict[str, str]``)
 * #8945: when transforming typed fields, call the specified role instead of
   making an single xref. For C and C++, use the ``expr`` role for typed fields.
+* #9322: KeyError is raised on PropagateDescDomain transform
 
 Testing
 --------

sphinx/transforms/post_transforms/__init__.py

@@ -263,7 +263,8 @@ class PropagateDescDomain(SphinxPostTransform):
 
     def run(self, **kwargs: Any) -> None:
         for node in self.document.traverse(addnodes.desc_signature):
-            node['classes'].append(node.parent['domain'])
+            if node.parent.get('domain'):
+                node['classes'].append(node.parent['domain'])
 
 
 def setup(app: Sphinx) -> Dict[str, Any]: