Stop empty state updates from `CronjobController`

**What is this about?**

Findings - Using [redux-devtools-expo-dev-plugin](https://github.com/matt-oakes/redux-devtools-expo-dev-plugin), we found some areas that should be optimized.

- `CronjobController` is also triggering many state updates. None of them includes any states. The controller needs to prevent these empty state updates from triggering OR we need to unsubscribe to the controller if it isn't meant to provide state

Video showing empty states on `CronjobController` triggered updates
https://github.com/user-attachments/assets/cc6f6cfb-0e9d-498a-a3c6-f53a163f3f7b

**Technical Details**

- Prevent empty state updates from triggering on `CronjobController`
- If state is not expected of the controller, we should unsubscribe from the state updates event

**Acceptance Criteria**

- `CronjobController` should not produce empty state updates

**Stakeholder review needed before the work gets merged**

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

**References**

Original findings - https://github.com/MetaMask/metamask-mobile/issues/15028#issuecomment-2892820535

Locate CronjobController and trace how its state updates are emitted, using the redux-devtools-expo-dev-plugin finding as context. Determine whether the controller should emit state at all, then verify that it no longer produces empty state updates and that the relevant automation tests pass.

Written by the indexing model from the issue text.