Error generating pull requests changelog on Github enterprise

when running: `gren changelog --generate --tags 5.0.0..4.2.0 --override --data-source prs --debug`

we're getting:
```
Options:
Tags: 5.0.0,4.2.0
Prefix: v
Template: [object Object]
Prerelease: false
Generate: true
Quiet: false
Override: true
Debug: true
Ignore labels: empty
Ignore issues with: duplicate,wontfix,invalid,help wanted
Ignore commits with: empty
Group by: [object Object]
Milestone match: Release {{tag_name}}
Changelog filename: CHANGELOG.md
Username: user
Repo: repo1
Token: 123321
Api url: https://github.enterprise.com/api/v3
Data source: prs
Include messages: commits
Ignore tags with: empty
Limit: 30

🤖 - Generate changelog file:
===================================
✔ Releases found: 3
✔ Tags found: 5.0.0, 4.2.0
⠸ Getting all merged pull requestsTypeError: Cannot read property 'length' of undefined
at /usr/local/lib/node_modules/github-release-notes/dist/Gren.js:1375:39
at Array.filter ()
at /usr/local/lib/node_modules/github-release-notes/dist/Gren.js:1374:42
at Array.reduce ()
at Gren._groupBy (/usr/local/lib/node_modules/github-release-notes/dist/Gren.js:1373:55)
at /usr/local/lib/node_modules/github-release-notes/dist/Gren.js:1566:104
at Array.map ()
at Gren._callee14$ (/usr/local/lib/node_modules/github-release-notes/dist/Gren.js:1563:57)
at tryCatch (/usr/local/lib/node_modules/github-release-notes/node_modules/regenerator-runtime/runtime.js:62:40)
at Generator.invoke [as _invoke] (/usr/local/lib/node_modules/github-release-notes/node_modules/regenerator-runtime/runtime.js:296:22)
```

Reproduce with the provided `gren changelog --generate --tags 5.0.0..4.2.0 --override --data-source prs --debug` command against GitHub Enterprise. Inspect `dist/Gren.js` around lines 1373-1375, where `_groupBy` accesses an undefined value while processing merged pull requests; done means the command completes without the TypeError and generates the changelog.

索引モデルが issue の本文から書いたものです。