ZEPPELIN-602 elasticsearch throws ArrayIndexOutOfBoundsException

### What is this PR for?
Fix for https://issues.apache.org/jira/browse/ZEPPELIN-602
"elasticsearch throws ArrayIndexOutOfBoundsException for interpreting an empty paragraph"

### What type of PR is it?
Bug Fix

### Todos
* [X] - Code : check cmd parameter

### Is there a relevant Jira issue?
ZEPPELIN-602

### How should this be tested?
Start elasticsearch interpreter with an empty paragraph

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? NO
* Is there breaking changes for older versions? NO
* Does this needs documentation? NO

Author: Bruno Bonnin <[email protected]>

Closes apache#646 from bbonnin/master and squashes the following commits:

05993d0 [Bruno Bonnin] Update unit tests and complete fix
378bf52 [Bruno Bonnin] Update ElasticsearchInterpreter.java
7651647 [Bruno Bonnin] ZEPPELIN-602 elasticsearch throws ArrayIndexOutOfBoundsException for interpreting an empty paragraph

Author: bbonnin

elasticsearch/src/main/java/org/apache/zeppelin/elasticsearch/ElasticsearchInterpreter.java

@@ -141,6 +141,10 @@ public void close() {
   @Override
   public InterpreterResult interpret(String cmd, InterpreterContext interpreterContext) {
     logger.info("Run Elasticsearch command '" + cmd + "'");
+ 
+    if (StringUtils.isEmpty(cmd) || StringUtils.isEmpty(cmd.trim())) {
+      return new InterpreterResult(InterpreterResult.Code.SUCCESS);
+    }
 
     int currentResultSize = resultSize;
 

elasticsearch/src/test/java/org/apache/zeppelin/elasticsearch/ElasticsearchInterpreterTest.java

@@ -198,4 +198,14 @@ public void testDelete() {
     assertEquals("11", res.message());
   }
 
+  @Test
+  public void testMisc() {
+
+    InterpreterResult res = interpreter.interpret(null, null);
+    assertEquals(Code.SUCCESS, res.code());
+
+    res = interpreter.interpret("   \n \n ", null);
+    assertEquals(Code.SUCCESS, res.code());
+  }
+
 }