Add handling for empty multi-line text field

Fix-up for https://github.com/zendesk/user_data_app/pull/37

Since empty multi-line fields are ‘null’ rather than an empty string, the last commit was attempting to call replace() on ‘null’ for empty fields and subsequently erroring out.

CC: @jbdietrich @benhass @maximeprades
@jbdietrich's suggestion will also handle `undefined` too.