handle null directory parameter

.vscode/numbered-bookmarks.json

@@ -1,19 +1,3 @@
 {
-	"bookmarks": [
-		{
-			"fsPath": "$ROOTPATH$/test/test.sh",
-			"bookmarks": [
-				-1,
-				57,
-				-1,
-				-1,
-				-1,
-				-1,
-				-1,
-				-1,
-				-1,
-				2
-			]
-		}
-	]
+	"bookmarks": []
 }

sri

@@ -88,6 +88,8 @@ while [ $# -gt 0 ]; do
             if [ -d "$2" ]; then
                 doDir=1
                 hashDir="${2%/}"
+            elif [ -z "$2" ]; then
+                displayError 1 "No directory specified."
             else
                 displayError 1 "Directory '$2' does not exist."
             fi