prompt: Avoid using unset variables

After `set -u` in Bash, the Git prompt would not be displayed and the
error `bash: GIT_PS1_COMPRESSSPARSESTATE: unbound variable` would be
printed, due to `GIT_PS1_COMPRESSSPARSESTATE` being unset by default
when it gets used.

This change sets a default value of an empty string to use when the
variable is unset to explicitly make the behaviour consistent between
`set -u` and `set +u`.

This fixes #2799

Signed-off-by: Chris Djali <krizdjali+github@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

contrib/completion/git-prompt.sh

@@ -431,8 +431,8 @@ __git_ps1 ()
 	fi
 
 	local sparse=""
-	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
-	   [ -z "${GIT_PS1_OMITSPARSESTATE}" ] &&
+	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE-}" ] &&
+	   [ -z "${GIT_PS1_OMITSPARSESTATE-}" ] &&
 	   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 		sparse="|SPARSE"
 	fi
@@ -541,7 +541,7 @@ __git_ps1 ()
 			u="%${ZSH_VERSION+%}"
 		fi
 
-		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
+		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE-}" ] &&
 		   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 			h="?"
 		fi