test(changelog): ensure error on missing changelog template filename

[ongdisheng]

Description

This PR adds a test to ensure the _export_template method raises a NotAllowed exception when the changelog template has no filename.

Related Code Fix PR: #1550
Close #1503

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (v4-9-0-test@05c1722). Learn more about missing BASE report.

Additional details and impacted files

@@              Coverage Diff               @@
##             v4-9-0-test    #1557   +/-   ##
==============================================
  Coverage               ?   98.02%           
==============================================
  Files                  ?       58           
  Lines                  ?     2680           
  Branches               ?        0           
==============================================
  Hits                   ?     2627           
  Misses                 ?       53           
  Partials               ?        0           

Flag	Coverage Δ
unittests	98.02% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.