completion: fix zsh check under bash with 'set -u'

Commit 06f44c3 (completion: make compatible with zsh) broke bash
compatibility with 'set -u': a warning was generated when checking
$ZSH_VERSION.  The solution is to supply a default value, using
${ZSH_VERSION-}.  Thanks to SZEDER Gábor for the fix.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>