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

首先使用 --data-source=prs-with-issues 复现报告中的 gren changelog 命令，并检查 dist/Gren.js:1760 中发生失败的堆栈位置。跟踪合并的 pull request 处理过程中 match 使用的 null 值；当命令在没有 TypeError 的情况下完成并生成 RELEASE_NOTES.md 时，即表示完成。

由索引模型根据 Issue 内容生成。