This change of behavior is the result of a bug fix for http://lists.gnu.org/archive/html/bug-bash/2015-08/msg00058.html, introduced in the following commit http://git.savannah.gnu.org/cgit/bash.git/commit/?id=f2d7e1a3bcbdec7ef09db71779d800237fbc58bb (read the changelog in CWRU/CWRU.chlog)
A couple of things here though: 1. The error is a non-fatal error 2. It is indeed an error to attempt to perform indirection on an empty or unset parameter. There is no good reason to revert the fix, considering that you were relying on unspecified behavior.
