Monorepo: Avoid creating empty PR

**Is your feature request related to a problem? Please describe.**
Kinda! We have a monorepo and it's creating a PR for the root (I guess) named `chore: release main` with 0 files on it.

I have the following release-please-config.json

```json
{
"include-component-in-tag": true,
"include-v-in-tag": false,
"tag-separator": "@",
"separate-pull-requests": true,
"pull-request-footer": "Al mergear esto va a deployar a PROD!",
"packages": {
"apps/web1": {
"component": "@app/web1",
"release-type": "node"
},
"apps/web2": {
"component": "@app/web2",
"release-type": "node"
},
"packages/sdk-js": {
"component": "@pkg/sdk",
"release-type": "node"
}
},
"plugins": [
{
"type": "node-workspace",
"updatePeerDependencies": true
}
],
"$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json"
}
```

I don't understand the reason that it's creating that empty PR if there is no "." packages in `release-please-config.json` file.

**A clear and concise description of what you want to happen.**

If there is no "." entry declared, it should create a PR for the root package.json.

**Add any other context or screenshots about the feature request here.**