tools/meson-vcs-tag: the third argument is optional

Follow-up for 1a71ac0.

Author: yuwata

tools/meson-vcs-tag.sh

@@ -6,7 +6,7 @@ set -o pipefail
 
 dir="${1:?}"
 fallback="${2:?}"
-version_tag="$3"
+version_tag="${3:-}"
 
 if [ -n "${version_tag}" ]; then
     # If -Dversion_tag= was used, just use that without further changes.