Support null values (type 0xff)

[zhuowei]

Some Splatoon byamls (e.g. content\Pack\Static.pack:Mush\MapInfo.byaml in update v64) contains tags with type 0xff and offset 0xffffff; I'm guessing they're null values, which do exist in regular YAML (http://yaml.org/type/null.html).

Not sure how to represent them in the XML version though - I'm currently printing them as "null", like <value AbnormalYPos="null", but that can't be imported back since "null" may legitimately occur in a string.

[Chadderz121]

I've had an attempt at adding support for null nodes in commit 17ec6a4. I chose to make null values separate elements rather than attributes, with type attribute set to "null". This should avoid ambiguity. If this doesn't work, please submit a patch, otherwise you can close this issue if it works first try.