test: add test for only witespace string in split_command

src/builtins/help.rs

@@ -53,4 +53,9 @@ mod tests {
 
     #[test]
     fn test_split_command_empty(){assert_eq!(split_command(""),vec!["",""])}
+
+    #[test]
+    fn test_split_command_only_space(){
+        assert_eq!(split_command("               "),vec!["",""]);
+    }
 }