Fixes T13147. In D19437, I changed this logic to support deleting the "" (empty string) token, but .pop() returns undefined, not null, if the list is empty and I didn't think to try deleting an empty input.
Fix the logic so we don't end up in a loop if the input is empty.
- In any browser, deleted all tokens in a tokenizer; then pressed delete again.
- Before: tab hangs in an infinte loop.
- After: smooth sailing.
Reviewers: amckinley, avivey
Reviewed By: avivey
Maniphest Tasks: T13147
Differential Revision: https://secure.phabricator.com/D19456