fix(firestore): allow null value to isEqualsTo & isNotEqualsTo in where() query

[russellwheatley]

Description

This PR uses a sentinel value to check whether isEqualTo or isNotEqualTo have been explicitly set to null.

The following PR also uses a sentinel value even though they cannot query with a null value:

• isLessThan
• isLessThanOrEqualTo
• isGreaterThan
• isGreaterThanOrEqualTo
• arrayContains

The following do not use the sentinel as their type differs from the rest (they're expecting a list of objects):

• arrayContainsAny
• whereIn
• whereNotIn

Related Issues

Closes #11874

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