chore: limit empty identity to remove (#30286)

Author: necusjz

src/azure-cli-core/azure/cli/core/aaz/_field_value.py

@@ -478,7 +478,7 @@ def to_serialized_data(self, processor=None, **kwargs):
 
         result = self._build_identity(calculate_data, result)
 
-        if not result:
+        if not result and calculate_data.get("action", None) == "remove":
             result = {"type": "None"}  # empty identity
 
         if not result and self._is_patch: