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

This is an update to this pull request https://github.com/mozilla/dxr/pull/374, rebased to a single commit, based on this suggestion on stack overflow: http://stackoverflow.com/questions/11396069/squash-to-only-one-proper-commit-for-github-pull-request

'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