Workaround openrc failure on zsh

The recent GIT_DEPTH change introduced a [[ ]] construct
which doesn't work for zsh 5.0.2.  Workaround it by tweaking the test.

The following is a demonstration to show how zsh behaves:

    % if [[ "" ]];then echo hoge;fi
    zsh: parse error near `]]'
    % if [[ "x" ]];then echo hoge;fi
    zsh: parse error near `]]'
    % if [[ -n "" ]];then echo hoge;fi
    % if [[ -n "x" ]];then echo hoge;fi
    hoge
    %

Closes-Bug: #1387943
Change-Id: Ia88de876dacb3664a7c3d8f5a035e8e50fddb678
Merge "Workaround openrc failure on zsh"