Prevent NullPointerException if "gatewayServer" does not exist

[OutOfBedlam]

What is this PR for?

Avoiding and preventing undesired NullPointerException during re-loading python interpreter
Since py4j is optional when PythonInterpreter starts, it should be optional too while it close.

What type of PR is it?

[Improvement]

What is the Jira issue?

• N/A

How should this be tested?

Reload PythongInterpreter on Zeppelin web "Interpreter" menu and check the log message if it complains NullPointerException

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? no
• Is there breaking changes for older versions? no
• Does this needs documentation? no

[bzz]

@OutOfBedlam thank you for catching this and contributing the improvement!

Before merging, could you please check that your code follows project code style guide esp regardin optional braces in Java?

CI failure seems un-related

Running org.apache.zeppelin.rest.ZeppelinSparkClusterTest
...
01:44:57,921 INFO org.apache.zeppelin.interpreter.remote.RemoteInterpreterProcess:143 - Run interpreter process [..//bin/interpreter.sh, -d, ../interpreter/spark, -p, 45995, -l, ..//local-repo/2BMVKYY9A]
..//bin/interpreter.sh: line 159: /home/travis/build/apache/zeppelin/run/zeppelin-interpreter-spark-travis-testing-worker-linux-docker-b1d10849-3384-linux-9.pid: No such file or directory
...
1:46:16,709 INFO org.apache.zeppelin.socket.NotebookServer:1116 - Job 20160614-014616_1473888867 is finished
Spark version detected 1.6.1
01:46:16,728 INFO org.apache.zeppelin.notebook.Paragraph:241 - run paragraph 20160614-014616_1069054480 using r org.apache.zeppelin.interpreter.LazyOpenInterpreter@3af54c62
No output has been received in the last 10 minutes, this potentially indicates a stalled build or something wrong with the build itself.
The build has been terminated

[bzz]

Thank you for the prompt responce!

CI passes now, proposed changes solve the issue and code looks great to me.

Merging if there is no further discussion.