fix(front-matter): handle empty frontMatter data

[timreichen]

fixes #6464

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.30%. Comparing base (2395035) to head (51fea13).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6481   +/-   ##
=======================================
  Coverage   95.29%   95.30%           
=======================================
  Files         575      576    +1     
  Lines       43279    43313   +34     
  Branches     6467     6480   +13     
=======================================
+ Hits        41243    41278   +35     
+ Misses       1995     1994    -1     
  Partials       41       41           

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[kt3k]

related #6393

[kt3k]

LGTM. Thanks!