Handle Podlove chapters without start times

[terminalchai]

A Podlove Simple Chapters element without a start attribute makes feedparser.parse() raise TypeError, including the minimized <psc:chapters<psc:chapter> input in #599 and otherwise well-formed RSS and Atom feeds.

Return None for an absent start time, matching the existing treatment of empty or invalid start strings. Chapter metadata and valid surrounding chapters are preserved.

Addresses the missing Podlove chapter start case in #599, rather than closing the entire malformed-feed report. Adds a changelog fragment and 13 test cases covering the reported input, RSS and Atom, strict and fallback parsing, and missing/empty/invalid start values.

Validation on Windows with Python 3.11:

• Before the fix: 5 new cases failed with TypeError; 8 compatibility cases passed. After the fix: all 13 pass.
• Full suite: 4,310 passed, 8 skipped.
• Black, isort, flake8, mypy (33 source files), and git diff --check pass.

Prepared with AI assistance; the diff and test results were reviewed before submission.