`init --non-interactive` does not populate the database or branch in credentials file

- Gel Version: 6.9+69ad2a3
- Gel CLI Version: Gel CLI 7.4.0+bb0c441
- OS Version: MacOS 15.5

Steps to Reproduce:

1. `gel init --server-instance example --database main --non-interactive`
2. cat "/edgedb/credentials/example.json"
3. See that `"database": null, "branch": null,`

This is breaking the gel-go client since after a non-interactive install the client cannot connect to the gel instance because it can't build the DSN:

```
2025/07/16 13:15:17 Error creating Gel client: gel.ConfigurationError: cannot read credentials for instance "example": gel.ConfigurationError: cannot parse credentials in "/Users/redacted/Library/Application Support/edgedb/credentials/example.json": `database` must be a string
exit status 1
```

Workaround is to manually build the DSN from the credentials file json first and set `GEL_DSN` before launching.