BUGZILLA-1111383 Don't assume that 'kind' is a valid key

'kind' is not necessarily emitted during a declDef record creation,
so we should provide a default value of None to avoid crashing.

https://bugzilla.mozilla.org/show_bug.cgi?id=1111383
Oh, I thought this was an error, but apparently get automatically uses None as a default. Should I make this noe '.get('kind', None)' or remove the None from the others? I slightly prefer the explicitness (also removes the question as to "why not just use '[]'"), but I guess the default was chosen for this reason.