Null handling by ProperContains, ProperIncludedIn, ProperIncludes, ProperIn evaluators.

[antvaset]

This fixes how the ProperContains, ProperIncludedIn, ProperIncludes, ProperIn evaluators handle nulls. The CQL specification states that:

• The T properly included in List<T> expression (ProperIn(T, List<T>) in ELM) evaluates to null if either argument is null (link).
• Same for List<T> properly included in List<T> (ProperIncludedIn(List<T>, List<T>) in ELM).
• List<T> properly includes T (ProperContains(List<T>, T) in ELM) evaluates to null if either argument is null (link).
• Same for List<T> properly includes List<T> (ProperIncludes(List<T>, List<T>) in ELM).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.72%. Comparing base (e00c396) to head (26c6921). Report is 8 commits behind head on master.

Files	Patch %	Lines
.../engine/elm/executing/ProperIncludesEvaluator.java	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1370 +/- ## ============================================ + Coverage 62.26% 63.72% +1.45% + Complexity 6951 2665 -4286 ============================================ Files 492 492 Lines 27727 27715 -12 Branches 5506 5502 -4 ============================================ + Hits 17265 17661 +396 + Misses 8198 7821 -377 + Partials 2264 2233 -31 ```

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[JPercival]

@antvaset - We have a goal of >=80% test coverage for new/updated code. Can you add tests for the Evaluators that gets the patch up to 80%?

[antvaset]

Hi @JPercival, I've squeezed in some more coverage. This is the most we can do I believe.