refactor: empty result plan.

kclvm/runner/src/lib.rs

@@ -104,7 +104,13 @@ pub fn exec_program(
     };
     // Exec result is a JSON or YAML string.
     let exec_result = match exec_result {
-        Ok(res) => res,
+        Ok(res) => {
+            if res.is_empty() {
+                return Ok(result);
+            } else {
+                res
+            }
+        }
         Err(res) => {
             if res.is_empty() {
                 return Ok(result);

kclvm/runner/src/runner.rs

@@ -288,7 +288,9 @@ impl KclvmRunner {
             warn_buffer,
         );
 
-        if n > 0 {
+        if n == 0 {
+            Ok("".to_string())
+        } else if n > 0 {
             let return_len = n;
             let s = std::str::from_utf8(&result[0..return_len as usize]).unwrap();
             wrap_msg_in_result(s)

test/grammar/misc/empty_plan/main.k

@@ -0,0 +1,2 @@
+# _a is a hidden attribute.
+_a = 1