fix(firestore): revert breaking change to where() API. null cannot be used for isEqualTo or isNotEqualTo in a query.

[russellwheatley]

Description

Reverting changes made in this PR: https://github.com/firebase/flutterfire/pull/11896/files

This constitutes a breaking change for users who are querying by passing wrapper values to all query options like below:

    ref.where(
      'field',
      isEqualTo: isEqualTo,
      isNotEqualTo: isNotEqualTo,
      isLessThan: isLessThan,
      isLessThanOrEqualTo: isLessThanOrEqualTo,
      isGreaterThan: isGreaterThan,
      isGreaterThanOrEqualTo: isGreaterThanOrEqualTo,
      arrayContains: arrayContains,
      arrayContainsAny: arrayContainsAny,
      whereIn: whereIn,
      whereNotIn: whereNotIn,
      isNull: isNull,
    );

Related Issues

fixes: #12130

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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.