Fix elemMatch null object array handling

[henyanagar]

Fixes #9103

Summary

This updates $elemMatch handling for arrays that contain both objects and null values.

Previously, $elemMatch decided whether to use rowFilter() based only on the first array element. If the first element was an object, later null entries were still passed into rowFilter(), which could throw a TypeError when resolving nested fields.

The updated logic checks each array element before applying object-field matching, so null entries are ignored for object-field $elemMatch selectors instead of throwing.

Testing

• Added a regression test for $elemMatch with an object array containing null.
• Verified the new test fails before the fix with:
  TypeError: Cannot read properties of null
• Verified after the fix:
  1 passing
• Verified the full elem-match test file:
  7 passing
• Ran ESLint on the changed source and test files.

[janl]

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