refactor(firestore)!: migrate to NNBD

[russellwheatley]

Description

Firestore NNBD (null-safety) migration.

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[Salakar]

Some leftover asserts also:

info: The operand can't be null, so the condition is always true. (unnecessary_null_comparison at [cloud_firestore] lib/src/collection_reference.dart:46)
info: The operand can't be null, so the condition is always true. (unnecessary_null_comparison at [cloud_firestore] lib/src/query.dart:56)

[fabriziocacicia]

Maybe the package version in the pubspec.yaml should be updated?

[Salakar]

Maybe the package version in the pubspec.yaml should be updated?

This is done automatically on master on publish.