Fix/14576 autodoc data signature

[itsvedantkumar]

Purpose

Hi, the bug was that autodoc doesn't build a signature for data objects. The problem was that its internal signature list is empty for them -- and when an autodoc-process-signature event handler returns a replacement signature, the code wrote it to index 0 of that empty list and ctashed with Indexerror (the docstring then got lost & a warning was printed.)

Fix: the change basically appends the handler's signature when the list is empty & behaves exactly as before when it isn't

Proof: a regression test is also included that reproduces the IndexError without the fix and passes with it; the full autodoc test suite was run with no new failures; CHANGES.rst and AUTHORS.rst entries are included

References

Fixes #14576

AI Disclosure

Claude Fable was used to develop the patch & the test under my direction -- I have reviewed the change & understand it, per the AI policy