fix(firestore, web): explicitly set null value on Firestore data object property value

[russellwheatley]

Description

A very bizarre exception occurring on Firestore web in release mode.

DocumentReference users = FirebaseFirestore.instance.collection('flutter-tests').doc('1');

final List<String?> media = List<String?>.filled(1, null);
//This will error even though `media.first` is a `null` object 
await users.set({'greatField2': media.first});

DocumentReference users = FirebaseFirestore.instance.collection('flutter-tests').doc('1');

//This will not error 
await users.set({'greatField2': null});

Explicity setting null fixes the issue. This is a hot fix, it appears to be an issue with how it is being evaluated by dart2js.

I've created a bug report on the Dart repo here.
I've created an issue which is tracked on our project board here.

Related Issues

fixes #9567

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