🐛 [cloud_Firestore] Update 4.14.0 CHANGELOG to clearly describe its BREAKING change

[kosukesaigusa]

Update 4.14.0 CHANGELOG to clearly describe its BREAKING change

According to the diff, since cloud_firestore version 4.14.0, aggregation count query returns int? instead of int, which is not compatible with previous versions.

So would it be better to clearly describe the BREAKING change for 4.14.0 CHANGELOG? (Otherwise, I would like to know the standard to judge what BREAKING changes are and others are not.)

And additionally, having doc comments to explain when the count property to return null is desired.

[darshankawar]

/cc @Lyokone

[ViniciusSossela]

It looks like a bug to return null...