docs: Add comment on GUID_NULL from IProvideClassInfo2 in FindOutgoingInterface.

Adds a comment to `FindOutgoingInterface` to explain that some COM servers may
return `GUID_NULL` from `IProvideClassInfo2.GetGUID` instead of a valid
interface IID.

This clarifies the subsequent error handling for this specific edge case.

Author: junkmd

comtypes/client/_events.py

@@ -121,6 +121,8 @@ def FindOutgoingInterface(source: IUnknown) -> type[IUnknown]:
         pass
     else:
         if guid == comtypes.GUID():
+            # Some COM servers, even if they implement `IProvideClassInfo2`,
+            # may return GUID_NULL instead of the default source interface's GUID.
             raise NotImplementedError("retrieved outgoing interface IID is GUID_NULL")
         # another try: block needed?
         try: