Fix ShardedClient#GetShardFor null-case (#742)
Fix ShardedClient#GetShardFor null-case (#742)

This resolves #742.

Common cases for IGuild being null on access are DMs (since they do not
belong to a guild) - this change resolves null guilds to shard zero,
where DMs are also received.

RogueException/Discord.Net,AntiTcb/Discord.Net