fix(GuildManager): #create throws when systemChannelFlags is undefined

[iShibi]

Please describe the changes this PR makes and why it should be merged:

This PR fixes a bug in GuildManager#create where it throws error if the optional systemChannelFlags param isn't provided. This happens because the method calls SystemChannelFlags#resolve without checking whether systemChannelFlags is defined or not. Since, it's an optional param, it can be undefined sometimes and passing undefined param to SystemChannelFlags#resolve always throws error.

Status and versioning classification:

• Code changes have been tested against the Discord API