Fix `Portal.run_in_actor()` returns `None` bug

Fixes the issue where if the main remote task returns `None`,
`Portal.result()` would erroneously wait again on the underlying feeder
mem chan since `None` was being used as the cache flag. Instead set the
flag as the channel uid and consider the result collected when set to
anything else (since it would be odd to return that value from a remote
task when you already can read it as part of portal/channel apis).

tractor/_portal.py

@@ -77,7 +77,7 @@ class Portal:
         # when this is set to a tuple returned from ``_submit()`` then
         # it is expected that ``result()`` will be awaited at some point
         # during the portal's lifetime
-        self._result: Optional[Any] = None
+        self._result: Optional[Any] = channel.uid
         # set when _submit_for_result is called
         self._expect_result: Optional[
             Tuple[str, Any, str, Dict[str, Any]]
@@ -128,6 +128,7 @@ class Portal:
         recv_chan: trio.abc.ReceiveChannel,
         resptype: str,
         first_msg: dict
+
     ) -> Any:
         assert resptype == 'asyncfunc'  # single response
 
@@ -158,7 +159,7 @@ class Portal:
 
         # expecting a "main" result
         assert self._expect_result
-        if self._result is None:
+        if self._result is self.channel.uid:
             try:
                 self._result = await self._return_once(*self._expect_result)
             except RemoteActorError as err: