fix(firebase_analytics): fixed type assertion that ignored nullable fields

[jorgecoca]

Description

In Firebase Analytics, when passing parameters to logPurchase, the signature allows for nullable values (Map<String, Object?>?). However, down the line, there is a method called _assertParameterTypesAreCorrect that asserts value types but ignored null values, creating a runtime issue.

More details in #12790

Related Issues

Fixes #12790

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