git-version-gen: avoid test -z portability glitch

git-version-gen: avoid test -z portability glitch

Autoconf warns that there are some broken shells where 'test -z "$x"'
gives 0 exit status if $x is ')'.  Since some of our strings come
from command-line arguments, and since git-version-gen is run on
end-user machines where sh might be broken, we should be robust to
abuse.  Some of the instances replaced here are provably safe,
and could not confuse even broken 'test', but it is easier to
replace all instances of test -[nz].

* build-aux/git-version-gen: Prefer portable test spelling, since
git-version-gen is run on more than just developer machines.

Signed-off-by: Eric Blake <eblake@redhat.com>