Fix non-matching defaults. (#5452) (#5454)

(cherry picked from commit f84a9bf932)

plugins/modules/files/xml.py

@@ -39,6 +39,7 @@ options:
     - The namespace C(prefix:uri) mapping for the XPath expression.
     - Needs to be a C(dict), not a C(list) of items.
     type: dict
+    default: {}
   state:
     description:
     - Set or remove an xpath selection (node(s), attribute(s)).