[ZEPPELIN-4124] Use optJSONArray to avoid exception

[bd2019us]

What is this PR for?

Avoid unexpected JSONException if no key exist.

What type of PR is it?

[Bug Fix | Improvement | Feature | Documentation | Hot Fix | Refactoring] BugFix

Todos

• [ ] - Task

What is the Jira issue?

• Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN/
• Put link here, and add [ZEPPELIN-Jira number] in PR title, eg. [ZEPPELIN-533]

https://issues.apache.org/jira/browse/ZEPPELIN-4124

How should this be tested?

• First time? Setup Travis CI as described on https://zeppelin.apache.org/contribution/contributions.html#continuous-integration
• Strongly recommended: add automated unit tests for any new or changed behavior
• Outline any manual steps to test the PR here.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update?
• Is there breaking changes for older versions?
• Does this needs documentation?

[zjffdu]

@bd2019us Thanks for the contribution, could you add unit test and update the PR description ?