fix: adding Null Handler Check and NPE unit test; rename variables

Fix issue causing intent without request handler to throw a NullPointerException (NPE), was not fixed in a historical PR: https://github.com/alexa/alexa-skills-kit-sdk-for-java/pull/312. isPresent is not the same as check is the Optional object itself is null. isPresent checks if the value of the Optional is null.
add unit test and rename variable.

Description

Fix Null Pointer Exception issue. For details, see this PR: https://github.com/alexa/alexa-skills-kit-sdk-for-java/pull/320
Add corresponding unit test for this change, which was missing and hence didn't catch the issue.
Rename variable name for better readability.

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

Pass all existing and new unit tests.

From the new unit test no_handler_response_returns_envelope_with_empty_response: Before the null check change:

java.lang.NullPointerException
    at com.amazon.ask.CustomSkill.invoke(CustomSkill.java:149)
    at com.amazon.ask.CustomSkill.invoke(CustomSkill.java:114)
    at com.amazon.ask.CustomSkill.invoke(CustomSkill.java:107)
    at com.amazon.ask.SkillTest.no_handler_response_returns_envelope_with_empty_response(SkillTest.java:92)...

After the null check change,can see the mock responseEnvelope contains

version = "1.0"
sessionAttributes = {HashMap@2337}  size = 0
userAgent = "ask-java/2.43.2 Java/1.8.0_322"
response = null

, which is consistent with the original PR Response JSON format.

Original PR and its testing process, see https://github.com/alexa/alexa-skills-kit-sdk-for-java/pull/320

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

fix: adding Null Handler Check and NPE unit test; rename variables #322