Fix #477 and fix #431: Prevent static constructor from being called by Dictionary deserializer

Created by: uwx

Summary

Fixes #477 (closed): DiscordGuild.GetRole returns null on .net core 3.0 linux arm

Fixes #431 (closed): System.MemberAccessException on initial startup with .NET Core 3.0 Preview builds

Details

As discussed in #431 (closed), this is caused by the addition of a static constructor in the Dictionary (or ConcurrentDictionary?) class in the BCL that only shows up in ARM64.

Changes proposed

• Add a static constructor check to the instantiation procedure in SnowflakeArrayAsDictionaryJsonConverter.ReadJson