Fix list equality logic

[antvaset]

This PR adds some fixes to the list equality logic.

1. Remove the early return statement inside CqlList.equal leading to e.g. this being true: {{'a','b'}, 'c'} = {{'a','b'},'d'}. Added the EqualWithSubListsTrue and EqualWithSubListsFalse tests for this case.

2. Fix null handling ("For list types, this means that equality returns true if and only if the lists contain elements of the same type, have the same number of elements, and for each element in the lists, in order, the elements are equal using equality semantics, with the exception that null elements are considered equal." - link). Added the EqualLiteralNull test to better cover this case.

3. The EqualNullNull test (XML-based) is adjusted to match the spec. The same change is proposed in Fix expected output for the EqualNullNull test cql-tests#38.

4. The test_Equal_BothListsHaveNull test (library-based) is also fixed to match the spec.

5. Because our XML-based tests use CQL's equality semantics to compare the expected to actual value of test expressions, it's now possible to un-skip the FlattenListNullAndNull test which previously wasn't passing because { null, null } = { null, null } didn't evaluate to true.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.69%. Comparing base (179c762) to head (f7238d5).

Files	Patch %	Lines
...s/cqf/cql/engine/elm/executing/EqualEvaluator.java	50.00%	0 Missing and 1 partial ⚠️
...va/org/opencds/cqf/cql/engine/runtime/CqlList.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1387      +/-   ##
============================================
- Coverage     63.69%   63.69%   -0.01%     
  Complexity     2666     2666              
============================================
  Files           492      492              
  Lines         27738    27738              
  Branches       5511     5511              
============================================
- Hits          17669    17668       -1     
  Misses         7828     7828              
- Partials       2241     2242       +1     

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