Fix so that returning nil as the autocomplete string works when committing the autocomplete text.

First of great autocomplete solution. Love it! I did run into some small things, hope to help improve this project with a PR. Let me know if I need to explain more/change anything to merge this in!

**The problem:**
When committing the autocomplete string while the datasource returned `nil` for the autocomplete string. The result is like `prefix(null)`.

**The fix:**
`[HTAutocompleteTextField refreshAutocompleteText]` already checks the length of the string instead of an `isEqualToString:` check. I've implemented this also in the [HTAutocompleteTextField commitAutocompleteText]` method. This solves the problem.