Restructure transport.lookup to avoid spurious mypy error

lookup() accepts None also, because it is intended to return the default.

dict.get() obviously accepts None, because it will not be
present and thus return the default. Apparently mypy wants
to be stricter than that.

Just restructure to be a bit plainer.

Author: ndevenish

src/workflows/transport/__init__.py

@@ -11,11 +11,12 @@
 default_transport = "PikaTransport"
 
 
-def lookup(transport: str) -> type[CommonTransport]:
+def lookup(transport: str | None) -> type[CommonTransport]:
     """Get a transport layer class based on its name."""
-    return get_known_transports().get(
-        transport, get_known_transports()[default_transport]
-    )
+    known_transports = get_known_transports()
+    if transport not in known_transports:
+        return known_transports[default_transport]
+    return known_transports[transport]
 
 
 def add_command_line_options(