dulwich: fallback to default client config in asyncssh vendor

- vendor should use ssh-agent or default IdentityFile(s) when no SSH key
  has been provided by the user
- encrypted (fallback) keys will always be passed to ssh-agent

dvc/scm/git/backend/dulwich/asyncssh_vendor.py

@@ -79,10 +79,11 @@ class AsyncSSHVendor(BaseAsyncObject, SSHVendor):
 
         conn = await asyncssh.connect(
             host,
-            port=port,
+            port=port if port is not None else (),
             username=username,
             password=password,
-            client_keys=[key_filename] if key_filename else [],
+            client_keys=[key_filename] if key_filename else (),
+            ignore_encrypted=not key_filename,
             known_hosts=None,
             encoding=None,
         )