Fix issue causing intent without request handler to throw a NullPointerException

Fix issue causing intent without request handler to throw a NullPointerException (NPE).

Description

An intent when added to the interaction model may not have a corresponding RequestHandler. While its acceptable that the response is null or empty as a result, without the null check update in the PR, a NullPointerException is thrown by the skill. This is because the Optional have "value" field and not having the RequestHandler causes the Optional object itself to be null. When the value field of a null object is accessed a NPE is thrown.

Motivation and Context

Avoid throwing an NPE in case of missing RequestHandler, instead handle the scenario more gracefully by providing an empty or null response.

Testing

Created a skill with an interaction model with an intent without a corresponding RequestHandler. After the update in this PR, the response from the skill was null whereas before this PR the skill threw a NPE.

Skill behavior before the change:

 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.Optional.orElse(java.lang.Object)' on a null object reference
    at com.amazon.ask.CustomSkill.invoke(CustomSkill.java:149)
    at com.amazon.ask.CustomSkill.invoke(CustomSkill.java:38)
    at com.amazon.ask.impl.AbstractSkill.execute(AbstractSkill.java:84)
    at com.amazon.ask.SkillStreamHandler.handleRequest(SkillStreamHandler.java:84)

Response JSON from skill after the change:

{"sessionAttributes":{},"userAgent":"ask-java/2.42.3 Java/0","version":"1.0"}

Note: This check existed before, but was removed in a previous commit due to a misunderstanding in the way the Optional value is handled. Commit: https://github.com/alexa/alexa-skills-kit-sdk-for-java/pull/286/commits/73296f63a12e7593dd070d3edaac54e6c9650734. Rest of the changes in the commit have been verified to work, just this minor change seems to have been unnecessary now.

Types of changes

[ x ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

[x ] My code follows the code style of this project
[x ] My change requires a change to the documentation
[ x] I have updated the documentation accordingly
[x ] I have read the README document
[x ] I have added tests to cover my changes
[ x] All new and existing tests passed

License

[x ] By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

alexa / alexa-skills-kit-sdk-for-java