Error: Cannot read properties of undefined (reading 'replace')

#### Environment details

- OS: glinux (Debian)
- Node.js version: v18.19.1
- npm version: 7.24.1
- `release-please` version: 16.14.3

#### Steps to reproduce

1. `npm exec release-please release-pr -- --repo-url=looker-open-source/sdk-codegen --debug --dry-run --token=${GITHUB_TOKEN}`

Get
```
❯ ts-node was not bumped, ignoring
❯ typescript was not bumped, ignoring
❯ url-loader was not bumped, ignoring
❯ uuid was not bumped, ignoring
❯ webpack was not bumped, ignoring
❯ webpack-bundle-analyzer was not bumped, ignoring
❯ webpack-merge was not bumped, ignoring
✔ Post-processing 20 in-scope candidates
❯ running plugin: LinkedVersions
✔ found 7 linked-versions candidates
TypeError: Cannot read properties of undefined (reading 'replace')
at safeBranchName ($HOME/release-please/node_modules/release-please/build/src/util/branch-name.js:214:23)
at BranchName.ofGroupTargetBranch ($HOME/release-please/node_modules/release-please/build/src/util/branch-name.js:63:93)
at LinkedVersions.run ($HOME/release-please/node_modules/release-please/build/src/plugins/linked-versions.js:128:58)
at Manifest.buildPullRequests ($HOME/release-please/node_modules/release-please/build/src/manifest.js:405:51)
at async Object.handler ($HOME/release-please/node_modules/release-please/build/src/bin/release-please.js:328:34)
```