[updater] Config fields required even if set at runtime

For [`updater`](https://tauri.app/plugin/updater/) it is possible to set all the required config fields at runtime: [endpoints](https://tauri.app/plugin/updater/#endpoints), [public key](https://tauri.app/plugin/updater/#public-key). However, it is still required that the `{ "plugins": { "updater": { "pubkey" } } }` field exists in the conf file.

You can get around this by setting the field to any value (e.g. `{ "plugins": { "updater": { "pubkey": "" } } }`), but it seems this should be unnecessary.