Bind empty string instead of "null" when onceText's value === null.

From the usability perspective, empty string is a better indicator of missing value than "null" string. In most use-cases IMHO this is desired behavior. The "null" string might seem like an error to the end-user.
Merge pull request #12 from xxccdef/master

Bind empty string instead of "null" when onceText's value === null.