fix(cloud_firestore): fix returning of transaction result

[Ehesp]

Description

This fixes an issue (specifically for web) whereby returning a none-standard value from a transaction threw an error. This change ignores the return value of the delegate transaction handler and instead only returns the user result.

Related Issues

• Fixes 🐛 [cloud_firestore] [web] Unexpected return from a transaction causes cryptic runtime error. #3495

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Breaking Change

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