[ZEPPELIN-6012] Fix NPE when the run-note request body carries no params

zeppelin-server/src/main/java/org/apache/zeppelin/rest/message/ParametersRequest.java

@@ -16,6 +16,7 @@
  */
 package org.apache.zeppelin.rest.message;
 
+import java.util.Collections;
 import java.util.Map;
 
 /**
@@ -29,7 +30,12 @@ public ParametersRequest(Map<String, Object> params) {
     this.params = params;
   }
 
+  /**
+   * Gson bypasses the constructor, so this field is null when the body carries no "params" entry.
+   *
+   * @return the parameters, or an empty map when none were supplied
+   */
   public Map<String, Object> getParams() {
-    return params;
+    return params == null ? Collections.emptyMap() : params;
   }
 }

zeppelin-server/src/test/java/org/apache/zeppelin/rest/NotebookRestApiTest.java

@@ -890,6 +890,33 @@ void testRunNoteWithParams() throws IOException, InterruptedException {
     }
   }
 
+  @Test
+  void testRunNoteWithoutParamsInBody() throws IOException {
+    LOGGER.info("Running testRunNoteWithoutParamsInBody");
+    String note1Id = null;
+    try {
+      note1Id = notebook.createNote("note1", anonymous);
+
+      // Running a note without form parameters is valid. Gson leaves ParametersRequest#params
+      // null both when the key is absent and when it is an explicit null, so neither body may fail.
+      for (String body : new String[] {"{}", "{\"params\":null}"}) {
+        CloseableHttpResponse post =
+                httpPost("/notebook/job/" + note1Id + "?blocking=true&isolated=true", body);
+        assertThat(post, isAllowed());
+        Map<String, Object> resp = gson.fromJson(
+                EntityUtils.toString(post.getEntity(), StandardCharsets.UTF_8),
+                new TypeToken<Map<String, Object>>() {}.getType());
+        assertEquals("OK", resp.get("status"), "Failed for request body: " + body);
+        post.close();
+      }
+    } finally {
+      // cleanup
+      if (null != note1Id) {
+        notebook.removeNote(note1Id, anonymous);
+      }
+    }
+  }
+
   @Test
   void testRunAllParagraph_FirstFailed() throws IOException {
     LOGGER.info("Running testRunAllParagraph_FirstFailed");