When rendering, skip missing keys

[xloem]

This is a very quick change to provide for rendering structures that are missing nonimportant keys. It simply skips them.

A change to provide default values for keys might be more thorough; unsure.

>>> baron.dumps([dict(type='print', value=[dict(type='associative_parenthesis', value=dict(type='string', value='"hello, world"'))])])
'print("hello, world")'