DateTimeComputed tries to compute from an empty value

#3025642: Remove the try/catch block from DateTimeComputed::getValue() revealed this bug.

If a date field is empty, trying to get the date computed property on it causes DateTimeComputed to try to create a date object, which causes DrupalDateTime to throw an exception. Currently that exception is caught, but the patch at #3025642: Remove the try/catch block from DateTimeComputed::getValue() re-throws it.

It doesn't make sense to attempt to create a date if we know there's no source value anyway, so DateTimeComputed should return early in that case.