Firebase.Firestore.DocumentSnapshot.ToDictionary() Crash if documentSnapshot.Exists == false #887

[REQUIRED] Please fill in the following fields:

• Unity editor version: 2020.1
• Firebase Unity SDK version: 6.16.1
• Source you installed the SDK: UPM (.unitypackage or Unity Package Manager)
• Problematic Firebase Component: Firestore (Auth, Database, etc.)
• Other Firebase Components in use: Auth, RemoteConfig (Auth, Database, etc.)
• Additional SDKs you are using: Google SignIn (Facebook, AdMob, etc.)
• Platform you are using the Unity editor on: Mac (Mac, Windows, or Linux)
• Platform you are targeting: Android (iOS, Android, and/or desktop)
• Scripting Runtime: IL2CPP (Mono, and/or IL2CPP)

[REQUIRED] Please describe the issue here:

documentSnapshot.ToDictionary() Crash if documentSnapshot.Exists == false

Documentation says

Returns | A Dictionary{String, Object} containing the document data or null if this is a nonexistent document.

Tested on 7.0.0, but it's totally broken (see other bugs).

Steps to reproduce:

Load a Document that does not exist

JNI DETECTED ERROR IN APPLICATION: can't call java.util.Set java.util.Map.keySet() on null object
...
CheckJNI::CallObjectMethodV(_JNIEnv*, _jobject*, _jmethodID*, std::__va_list)+72)
/lib/arm64/libFirebaseCppApp-6_16_1.so (firebase::firestore::FieldValueInternal::map_value() const+160)
...