Fixed ArrayUtils::pickRandom() when $rand=0.

Change-Id: Iba7db16a9ad08dd845fa1b4dd2bc02ffad8c5a32
Return empty string for trail/prefix if disabled

This is necessary so Parsoid can know when the API doesn't have the linktrail/
prefix feature and when the API is actually telling us that the trail/prefix
is/are disabled.

Bug: 41151
Change-Id: If0895d507fe1b473e42bceb135d4030106ded442
Merge "Return empty string for trail/prefix if disabled"
Merge "Fixed ArrayUtils::pickRandom() when $rand=0."