Exporting tests of test plan fails when there are invalid links

When there are invalid Test Plan and Test links, if the user uses the endpoint below to export the Test cases, it will fail with a Null Pointer Exception.

/rest/raven/1.0/api/testplan/{testplankey}/test

The root cause of this issue is due to the existence of invalid links between the Test Plan and its Tests - This can be caused, for instance, when changing the Issue Type of one Test issue while having Xray disabled.

If the user runs the Integrity Checker, it will report that there are invalid links which needs to be fixed. Running the fix should resolve the situation.

This endpoint should not fail, instead the valid Tests should be exported and this situation should be logged in the logs either in ERROR or WARN level.