twa_run: better protection against empty cycles

Fixes #337.

* spot/twaalgos/emptiness.cc, spot/twaalgos/emptiness.hh: Here.
* tests/python/except.py: Test it.
* NEWS: Mention the issue.
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passed debpkg-stable #3320

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passed mingw-shared #3318

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passed mingw-static #3319

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