git-prompt: work under set -u

git-prompt: work under set -u

Commit afda36dbf3 ("git-prompt: include sparsity state as well",
2020-06-21) added the use of some variables to control how to show
sparsity state in the git prompt, but implicitly assumed that undefined
variables would be treated as the empty string.  This breaks users who
run under 'set -u'; fix the code to be more explicit.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>