feat: Handle GUID_NULL for outgoing interface IID in FindOutgoingInterface.

When `IProvideClassInfo2` returns `GUID_NULL` as the outgoing interface IID,
`FindOutgoingInterface` now raises a `NotImplementedError`. This addresses
cases where some COM servers (like `htmlfile`) return a null GUID instead
of the default source interface's GUID.

A new test case `test_retrieved_outgoing_iid_is_guid_null` has been added
to `test_eventinterface.py` to verify this behavior.

Author: junkmd

comtypes/client/_events.py

@@ -120,6 +120,8 @@ def FindOutgoingInterface(source: IUnknown) -> type[IUnknown]:
     except COMError:
         pass
     else:
+        if guid == comtypes.GUID():
+            raise NotImplementedError("retrieved outgoing interface IID is GUID_NULL")
         # another try: block needed?
         try:
             interface = comtypes.com_interface_registry[str(guid)]

comtypes/test/test_eventinterface.py

@@ -112,16 +112,13 @@ def test_nondefault_eventinterface(self):
 
 
 class Test_MSHTML(ut.TestCase):
-    def test(self):
+    def test_retrieved_outgoing_iid_is_guid_null(self):
         doc = CreateObject("htmlfile")
         sink = object()
         # MSHTML's HTMLDocument (which is what `CreateObject('htmlfile')`
         # returns) does not expose a valid default source interface through
         # `IProvideClassInfo2`.
-        # Specifically, it returns `GUID_NULL` as its default source interface,
-        # which results in a `KeyError` when `GetEvents` tries to look it up in
-        # the interface registry.
-        with self.assertRaises(KeyError, msg="{00000000-0000-0000-0000-000000000000}"):
+        with self.assertRaises(NotImplementedError):
             GetEvents(doc, sink)