Fix printing of empty Ands and Ors

[francescofuggitti]

Fixes

The empty And() formula resulted in the TrueFormula (true), which is not part of PDDL and, therefore, not recognized by planners. The same happened for the empty Or() formula.

Proposed changes

In this PR, we fix such behavior and simplify the code structure by removing the TrueFormula() and the FalseFormula(). We also update the rest of the code.

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

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• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

[haz]

Aye, it all looks perfect to me! Can't say I can ever recall seeing the empty or in PDDL, but it all makes sense.

[ssardina]

For the records, TRUE would be And() and FALSE would be Or() after this PR right? (I used to use TrueFormula which is not there anymore after this PR)

[haz]

@ssardina I believe so, yep!