Make unavailable default to None.
Make unavailable default to None.

rapptz/discord.py,jhgg/discord.py,khazhyk/discord.py,variablehair/Eggplantato,Rapptz/discord.py,imayhaveborkedit/discord.py,Harmon758/discord.py,Theoretical/UselessBot,mgardne8/discord.py,lgkern/PriestPy,Harmon758/discord.py