Fix for empty TRX TestDefinitions

[OlesGalatsan]

This PR is a fix for #64
Currently, this action only checks if TestDefinitions are not defined, but it is possible that it will just be empty:

<?xml version="1.0" encoding="utf-8"?>
<TestRun id="f4386078-5371-420d-b9c6-122f1e77f109" name="test" xmlns="http://microsoft.com/schemas/VisualStudio/TeamTest/2010">
  <Times creation="2025-05-13T12:53:18.295323Z" queuing="2025-05-13T12:53:18.295323Z" start="2025-05-13T12:53:18.295323Z" finish="2025-05-13T12:53:18.4362393Z" />
  <TestSettings name="default" id="f25a2621-1af4-4d50-a859-619a3039691e">
    <Deployment runDeploymentRoot="test" />
  </TestSettings>
  <Results />
  <TestDefinitions />
  <TestEntries />
  <TestLists>
    <TestList name="Results Not in a List" id="8C84FA94-04C1-424b-9868-57A2D4851A1D" />
    <TestList name="All Loaded Results" id="19431567-8539-422a-85d7-44ee4e166bda" />
  </TestLists>
  <ResultSummary outcome="Completed">
    <Counters total="0" executed="0" passed="0" failed="0" error="0" timeout="0" aborted="0" inconclusive="0" passedButRunAborted="0" notRunnable="0" notExecuted="0" disconnected="0" warning="0" completed="0" inProgress="0" pending="0" />
  </ResultSummary>
</TestRun>

In order to fix it, we need to check if td.UnitTest exists and is an array for it to be then parsed.
Test run: https://github.com/OlesGalatsan/MstTrx/actions/runs/15017567979