change our tokenize to yield empty tokens

Our tokenize function currently skips empty tokens.  This means we
incorrectly accept invalid syntax with our seccomp filter like:
	close: arg0 == 1 |||||| arg0 == 2

Change the tokenizer helper to yield an empty string in this case so
we can correctly detect & reject these.  We don't have any scenarios
where we actually want to allow empty strings currently either (and
if we did, the callers could check themselves).

Bug: None
Test: unittests pass
Change-Id: I282e4e4544a24c0e5a7036b693429bdd209339cf
4 files changed