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.
10 jobs for next in 132 minutes and 40 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
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passed arch-gcc-glibcxxdebug #3147

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passed debian-gcc-snapshot #3145

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passed debian-stable-gcc #3143

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passed debian-unstable-gcc-coverage #3144

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passed debpkg-stable #3150

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passed debpkg-unstable #3151

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passed mingw-shared #3148

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  Publish
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dput

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