None metadata is not allowed

This fixes recent launch failures with error `Launching server failed: TypeError("'NoneType' object does not support item assignment",)`
Yeah, we only discovered it because we were testing something for mimic.  Try passing an into or something to nova.  They say '255 bytes', and I guess they think that implies a string?  But passing in an int or array or another object returns a 400 saying that the value is not a string.