fix: Fix output for str.extract_groups with empty string pattern

[kdn36]

Partial fix for #23663.

The engine was modified: for the empty string pattern (""), it now generates an empty Struct, rather than a Struct containing the 'missing' value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.33%. Comparing base (fcee607) to head (a9967d6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23698      +/-   ##
==========================================
+ Coverage   81.31%   81.33%   +0.01%     
==========================================
  Files        1643     1643              
  Lines      223182   223130      -52     
  Branches     2837     2837              
==========================================
- Hits       181490   181477      -13     
+ Misses      40993    40954      -39     
  Partials      699      699              

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