R1:31ee49b14d93

Fix Javascript busy loop when trying to delete tokens from an empty tokenizer

Summary:
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.

Test Plan:
- In any browser, deleted all tokens in a tokenizer; then pressed delete again.
- Before: tab hangs in an infinte loop.