Charm stays in `waiting` mode while waiting for `object-storage` relation

### Bug Description

While the `mlflow-server` charm is not related via the `object-storage` interface, the charm status is `waiting` status with the message `Waiting for object-storage relation data`.

For what it's worth the [Juju status documentation](https://documentation.ubuntu.com/juju/latest/reference/status/index.html) clearly states that:
> `Waiting` status is for healthy, but waiting for another application to be Active, or a package installation
> `Blocking` status is a problem that needs manual intervention, including having a **missing relation**

### To Reproduce

Deploy the charm without relating with `minio`:
```
juju deploy mlflow-server --trust
juju status mlflow-server
```

### Environment

.

### Relevant Log Output

```shell
Unit Workload Agent Address Ports Message
mlflow-server/0* waiting idle 10.1.90.161 Waiting for object-storage relation data
```

### Additional Context

_No response_

Reproduce the issue with `juju deploy mlflow-server --trust` and `juju status mlflow-server` without a `minio` relation. Trace the charm's handling of missing object-storage relation data and verify that the resulting status is Blocking rather than Waiting, with an appropriate message.

Written by the indexing model from the issue text.