Fix BelongsToMorphed not clearing morph type on null relation

[roxblnfk]

🔍 What was changed

Fix BelongsToMorphed not clearing morph type when relation is set to null

When setting a BelongsToMorphed relation to null, only the foreign key(parent_id) was being cleared while the morph discriminator (parent_type)retained its previous value. This left the database in an inconsistentstate with parent_type pointing to a role but parent_id being NULL.

The fix makes BelongsTo::setNullFromRelated() overridable (private → protected) and overrides it in BelongsToMorphed to also register the morphKey as null.

📝 Checklist

• How was this tested:
  • Unit tests added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.75%. Comparing base (ea1fbb5) to head (7391f8b).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #555   +/-   ##
=========================================
  Coverage     91.75%   91.75%           
- Complexity     1962     1963    +1     
=========================================
  Files           131      131           
  Lines          5031     5034    +3     
=========================================
+ Hits           4616     4619    +3     
  Misses          415      415           

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