Fix null handling in preg_replace calls inside normalize_content()

[juanfra]

What?

This PR fixes a potential bug where preg_replace() failures could silently corrupt content in the normalize_content() function. It's an edge case.

Why?

If any of these preg_replace() calls failed and returned null, the code would continue with null being cast to an empty string in subsequent operations. This could silently corrupt content without any indication of an error.

How?

Added null coalescing operator to all three preg_replace() calls in normalize_content().

Not sure if we want to document the @since for any modifications. If that's the case, please let me know, and I'll be happy to make the addition.

[github-actions]

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Co-authored-by: juanfra <[email protected]>
Co-authored-by: jeffpaul <[email protected]>
Co-authored-by: dkotter <[email protected]>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.33%. Comparing base (9a4845d) to head (fd0d510).
⚠️ Report is 20 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #177      +/-   ##
=============================================
- Coverage      50.91%   50.33%   -0.58%     
+ Complexity       375      366       -9     
=============================================
  Files             27       26       -1     
  Lines           1974     1951      -23     
=============================================
- Hits            1005      982      -23     
  Misses           969      969              

Flag	Coverage Δ
unit	50.33% <100.00%> (-0.58%)	⬇️

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