test/lib/create-git-repo: fix empty var expansion

When `amend` is empty, expand it to nothing instead of an empty
string (which causes a git error).

test/lib/create-git-repo.sh

@@ -2,14 +2,14 @@
 (
   # shellcheck disable=SC2154,SC2164
   cd "$scriptDir/.."
-  amend=--amend
+  amend=(--amend)
 
   if [[ ! -e .git ]] || ! git rev-parse HEAD 2>/dev/null; then
     git init
-    amend=
+    amend=()
   fi
   git add .
   if ! git diff --quiet --cached; then
-    git commit -a "$amend" -m -
+    git commit -a "${amend[@]}" -m -
   fi
 ) >/dev/null