githooks: fix pre-commit hook to allow empty source

.githooks/pre-commit

@@ -40,6 +40,7 @@ abuild_local_sources() {
 	set -eu
 
 	status=0
+	: ${source:=""}
 	for src in $source; do
 		# Skip remote sources.
 		case "$src" in */*) continue;; esac