Validate Query formatter - handle undefined

I noticed when I chose the Filter agg in the agg builder that this would throw an unhandled error - apparently there was no text being passed in.

Now, if `toUser` gets undefined, it returns an empty string. Added test to confirm.
Personal preference, `text == null` rather than `_.isUndefined` since `null` behaves very similarly to `undefined`, and will also break this function.

Also, it might be a good idea to use `'' + text` rather than `text.toString()` in order to prevent similar errors.