[fix] error when adding tags with keyboard. Fixes #4394

This fixes the issue described in:
https://github.com/joomla/joomla-cms/issues/4394

This was introduced in: https://github.com/joomla/joomla-cms/commit/88ed53e960c70830d6cce2a16add1dec2a79808e#diff-0fe9e3814af6ea886d29a21726738a2aR166

Apparently minTermLength fix from @davidhurley's didn't include a default value for the setting. So it was using 0 as minimum length (until you save the `com_params` config). I added an extra `this.value` check to be sure that an empty tag is never going to be added even if someone manages to use 0 as `minTermLength` (which should not be possible because the minimum in config is 1).