Silence error messages from tagged journal entries ajax script

Bug 1557896: When you first click in Select2, it fires off an
aqax request with no "q" value (because there's no query string
yet). We return a "required parameter missing" error message in
a JSON object, which causes a Javascript warning in the browser.

Better to just treat it like an empty q, instead of a missing q.

behatnotneeded - unable to test for issues in console

Change-Id: I81c4cfe77c51fce735cfd2628aaed1e40cc35940