Check for empty receivers after expired weakrefs are pruned

Signed-off-by: Sahas Subramanian <sahas.subramanian@proton.me>

Author: shsms

src/frequenz/channels/_broadcast.py

@@ -381,10 +381,6 @@ async def send(self, message: _T, /) -> None:
             raise SenderError("The channel was closed", self) from ChannelClosedError(
                 self._channel
             )
-        if self._channel._auto_close_enabled and len(self._channel._receivers) == 0:
-            raise SenderError("The channel was closed", self) from ChannelClosedError(
-                self._channel
-            )
         self._channel._latest = message
         stale_refs = []
         for _hash, recv_ref in self._channel._receivers.items():
@@ -395,6 +391,10 @@ async def send(self, message: _T, /) -> None:
             recv._enqueue(message)
         for _hash in stale_refs:
             del self._channel._receivers[_hash]
+        if self._channel._auto_close_enabled and len(self._channel._receivers) == 0:
+            raise SenderError("The channel was closed", self) from ChannelClosedError(
+                self._channel
+            )
         async with self._channel._recv_cv:
             self._channel._recv_cv.notify_all()
         # pylint: enable=protected-access