FIXME: Make TProtocol Optional[TProtocol] to keep types consistent (#770)

* FIXME: Make TProtocol Optional[TProtocol] to keep types consistent

* correct test case of test_protocol_muxer

* add newsfragment

* unit test added

---------

Co-authored-by: Manu Sheel Gupta <manusheel.edu@gmail.com>

libp2p/security/security_multistream.py

@@ -17,6 +17,9 @@ from libp2p.custom_types import (
 from libp2p.peer.id import (
     ID,
 )
+from libp2p.protocol_muxer.exceptions import (
+    MultiselectError,
+)
 from libp2p.protocol_muxer.multiselect import (
     Multiselect,
 )
@@ -104,7 +107,7 @@ class SecurityMultistream(ABC):
         :param is_initiator: true if we are the initiator, false otherwise
         :return: selected secure transport
         """
-        protocol: TProtocol
+        protocol: TProtocol | None
         communicator = MultiselectCommunicator(conn)
         if is_initiator:
             # Select protocol if initiator
@@ -114,5 +117,7 @@ class SecurityMultistream(ABC):
         else:
             # Select protocol if non-initiator
             protocol, _ = await self.multiselect.negotiate(communicator)
+        if protocol is None:
+            raise MultiselectError("fail to negotiate a security protocol")
         # Return transport from protocol
         return self.transports[protocol]