`.canonical_label()` errors on bipartite graphs when `partition=` is not specified

[maxale]

Steps To Reproduce

sage: G = BipartiteGraph( matrix([[1,1],[1,1]]) )
sage: G.canonical_label()
[...]
TypeError: itertools.chain() argument after * must be an iterable, not NoneType

Expected Behavior

Should work.

Actual Behavior

TypeError

Additional Information

Specifying partition= eliminates the issue:

sage: G.canonical_label(partition=G.bipartition())
Bipartite graph on 4 vertices

Perhaps, G.bipartition() should be the default value for partition= in the case of bipartite graphs.

Environment

• OS: Ubuntu 24.04.1 LTS
• Sage Version: 10.5.beta6

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[dcoudert]

good catch. I pushed a possible fix in #38842.