Error when using `prs-with-issues`

Using `prs-with-issues` as the data source resulted in an error.

`gren changelog --generate --override --changelog-filename=RELEASE_NOTES.md --tags=v15.1.1-Preview.13..v15.0.4 --data-source=prs-with-issues`


```
Getting gren config from local file C:\Users\MartinHinshelwoodNKD\source\repos\azure-devops-migration-tools\.grenrc.yml

🤖 - Generate changelog file:
===================================
√ Releases found: 30
√ Tags found: v15.1.1-Preview.13, v15.0.4
/ Getting all merged pull requestsTypeError: Cannot read properties of null (reading 'match')
at _context17.t1 (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\dist\Gren.js:1760:59)
at Array.map ()
at Gren._callee17$ (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\dist\Gren.js:1773:77)
at tryCatch (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\node_modules\regenerator-runtime\runtime.js:62:40)
at Generator.invoke [as _invoke] (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\node_modules\regenerator-runtime\runtime.js:296:22)
at prototype. [as next] (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\node_modules\regenerator-runtime\runtime.js:114:21)
at step (C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\node_modules\babel-runtime\helpers\asyncToGenerator.js:17:30)
at C:\Users\MartinHinshelwoodNKD\AppData\Roaming\npm\node_modules\github-release-notes\node_modules\babel-runtime\helpers\asyncToGenerator.js:28:13
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
/ Getting all merged pull requests
```

Never finishes

Start by reproducing the reported gren changelog command with --data-source=prs-with-issues and inspect the failing stack location in dist/Gren.js:1760. Trace the null value used by match during merged pull request processing; done means the command completes without the TypeError and generates RELEASE_NOTES.md.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.