Fix type conversion operator tests for malformed inputs. Enable tests for Boolean and null literals.

[antvaset]

• Fixing the ToInteger(String), ToDateTime(String), ToTime(String) tests for malformed string arguments (see spec for ToInteger, ToDateTime, ToTime).
• Enabling the Boolean and null literal tests (key change at antvaset@d4dd873#diff-223279de96f0d1e72a0e8cf595773009f4668f46512388a3b6f9f7ad404749d2R147).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.26%. Comparing base (e00c396) to head (529a2f8).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1371      +/-   ##
============================================
- Coverage     62.26%   62.26%   -0.01%     
- Complexity     6951     6953       +2     
============================================
  Files           492      492              
  Lines         27727    27731       +4     
  Branches       5506     5508       +2     
============================================
+ Hits          17265    17266       +1     
- Misses         8198     8200       +2     
- Partials       2264     2265       +1     

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[JPercival]

@brynrhodes - Bump