Omit block for test failure details in annotation when there's none

[mokagio]

Not all test nodes failures in the JUnit report have a value.

E.g. from the failures in this build:

<testcase classname="StatsTests" name="testInsightsStatsLoadProperly()" time="131.52018702030182">
  <failure message="failed: caught error: "timedOut" (#CharacterRangeLen=0)"> </failure>
</testase>

In such cases, the failure annotation looks a bit odd:

The changes in this PR make the annotation look like this:

---

• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

We've been wondering with how to best test these changes and other changes. See internal ref p1685981991334859-slack-C02KLTL3MKM

It's crucial for the long term health of this project that we find an answer. However, I think the scope is broader than this PR and therefore I'm going to merge in the meantime to roll out the improvement.

On that note, I had a play around and have a proof of concept for using RSpec to test part of the code edited here: #64