map the null code point to U+FFFD when decoding CSS escapes

[itecz26]

the escape decoder _replace_unicode folds surrogate halves and out-of-range code points to U+FFFD but not the null code point, so a \0 escape (or literal \0 in an identifier/string) decodes to a NUL that leaks into decoded names, string values and the generated XPath; the CSS Syntax spec (§4.3.7) treats zero the same as surrogates, so this adds codepoint == 0 to the same guard #164 introduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (9790903) to head (440813d).

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #189      +/-   ##
===========================================
- Coverage   100.00%   99.79%   -0.21%     
===========================================
  Files            3        3              
  Lines          966      966              
  Branches       155      155              
===========================================
- Hits           966      964       -2     
- Misses           0        1       +1     
- Partials         0        1       +1     

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