bsdgrep: revise test case which will soon become a failure

Work in progress (D10315) is going to make egrep_empty_invalid an
actually invalid regex, to be consistent with the equivalent BRE "{"
behavior, when using regex(3).

Any non-0 exit value is acceptable, depending on how the installed grep
interprets the expression. GNU grep interprets it as non-matching, and
in the future BSD grep will interpret it is an error.

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem, ngie
Differential Revision: https://reviews.freebsd.org/D10572`