Fix mypy error

This bug is old, but it only triggers if there are no UIDs, which is why
testing didn't reveal it.  I suspect old versions of mypy just did not
catch the bug.

splitgpg2/__init__.py

@@ -819,7 +819,9 @@ async def setkeydesc(self, keygrip: bytes) -> None:
             assert key is not None, 'no key?'
 
             desc = b'%s\nFingerprint: %s%s' % (
-                    (b'UID: ' + key.first_uid.split(b'\n')[0]) if key.first_uid is not None else '',
+                    (b'UID: ' + key.first_uid.split(b'\n')[0])
+                    if key.first_uid is not None
+                    else b'',
                     key.fingerprint,
                     subkey_desc)