fix: Fix rule's null value comparisons for is and isNot

[bvanelli]

The JS implementation can be seem on the file condition.ts. On Actual rules, most None evaluations will use the guard:

if (fieldValue == null) {
    return false;
}

When using is and isNot comparing with None values, there is an exception since it uses fieldValue === this.value and fieldValue !== this.value. We should allow the comparison to evaluate with the nullable field instead.

Another bug fix was comparing a null to a string object. There is a specific guard for this behavior:

if (type === 'string') {
    fieldValue ??= '';
}

Here, the correct option is to convert both sides to an empty string, then use the compare function instead.

Closes #197

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.62%. Comparing base (0039ad2) to head (9abdbec).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
- Coverage   97.64%   97.62%   -0.03%     
==========================================
  Files          21       21              
  Lines        3231     3237       +6     
==========================================
+ Hits         3155     3160       +5     
- Misses         76       77       +1     

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