Fix for non-existing test folder, resulting in null listOfFiles

[MrLurch81]

Purpose

When the default test folder (in build-artifacts, since Integration Studio 8.2.0) is not present, the plugin throws an exception. However, it should just return that no test cases were found, similar to when the test folder does exist but is empty.
Fixes the issue #114

Approach

Add a check to see if listOfFiles is not null before trying to read all files in listOfFiles.

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[MrLurch81]

Hi @GDLMadushanka and @SanojPunchihewa,
I see that you have previously approved changes in this project. Any chance either of you could review this PR so it is no longer required to disable the maven tests?