fix(front-matter): handle empty frontMatter data

[timreichen]

Ref: #6464

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.41%. Comparing base (bcb30bb) to head (c1fb358).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6481      +/-   ##
==========================================
- Coverage   95.67%   95.41%   -0.27%     
==========================================
  Files         574      556      -18     
  Lines       43182    43040     -142     
  Branches     6470     6411      -59     
==========================================
- Hits        41314    41065     -249     
- Misses       1829     1933     +104     
- Partials       39       42       +3     

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