publicode.yml with maintenance internal but no contacts validates succesfully

According to the documentation contacts must be present if maintenance type is internal: https://yml.publiccode.tools/schema.core.html?highlight=contacts#key-maintenance-contacts

As seen in the following screenshot, a publiccode.yml without contacts for maintenance type internal validates successfully in the editor.

then, italia/publiccode-crawler:latest then fails to validate the file with the following message:

```
error: maintenance.contacts: contacts is a required field when \"type\" is \"internal\"\n",
```