Fix a bug when the automaton contain trivial acceptance conditions

[lou1306]

Proposed changes

The validation step would fail on "trivial" acceptance conditions such as 0 t or 0 f

Fixes

• I added a HOA automaton and accompanying test case that exposes the bug.
• I fixed the bug by modifying parsers.HOATransformer.acceptance

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further comments

I am opening the PR against master because develop is 1 commit behind it. If you align develop, please let me know and I'll be happy to re-submit.