Restore Interface Contract in DOM.php

[jakejackson1]

In https://github.com/GravityPDF/querypath/commit/35e92660d680f18a332eec10fdc8a6ecc7596fb7 the $selector property was changed from null to '', which broke the Query interface contract.

Restore the contract, instead of updating the interface, to prevent a breaking change.

Pull Request type

Please check the type of change your PR introduces:

• [X] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Build-related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

What is the current behavior?

The interface and the class implementing it were at odds with the type hints.

What is the new behavior?

Update the class so it matches the interface.

Does this introduce a breaking change?

• [ ] Yes
• [X] No

[codecov[bot]]

Codecov Report

Merging #41 (86e4bc2) into main (d264d91) will decrease coverage by 0.00%. The diff coverage is 83.33%.

@@             Coverage Diff              @@
##               main      #41      +/-   ##
============================================
- Coverage     88.40%   88.39%   -0.01%     
  Complexity     1349     1349              
============================================
  Files            26       26              
  Lines          3088     3086       -2     
============================================
- Hits           2730     2728       -2     
  Misses          358      358              

Impacted Files	Coverage Δ
src/qp_functions.php	66.66% <ø> (ø)
src/QueryPath.php	68.75% <75.00%> (ø)
src/DOM.php	89.74% <100.00%> (-0.11%)	:arrow_down:

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