Fix a bug when related fields were not filled in BelongsToMorphed

[roxblnfk]

🔍 What was changed

Fixed a bug where the linking fields were not mapped when saving a BelongsToMorphed relation by assigning a related entity only.

Can't reproduce in tests

📝 Checklist

• Closes #
• How was this tested:
  • Tested manually
  • Unit tests added

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.73%. Comparing base (72ee07e) to head (ca53d95).
⚠️ Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
src/Relation/BelongsTo.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #550   +/-   ##
=========================================
  Coverage     91.73%   91.73%           
  Complexity     1924     1924           
=========================================
  Files           123      123           
  Lines          4958     4958           
=========================================
  Hits           4548     4548           
  Misses          410      410           

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