NO-ISSUE: Ensure that openshift_version isn't empty

We found a situation where the `openshift_version` column of the
clusters table is empty. We haven't been able to understand why, but it
results in an error when running validations. For example, the
dual-stack VIP validation produces a false negative:

```
2024-09-11T05:39:04.055138029Z time="2024-09-11T05:39:04Z"
level=error
msg="Cluster 686c7849-8d68-4340-b81d-9add8c4261fe failed VIP validations"
func="github.com/openshift/assisted-service/internal/bminventory.(*bareMetalInventory).validateAndUpdateClusterParams"
file="/remote-source/assisted-service/app/internal/bminventory/inventory.go:1889"
error="dual-stack VIPs are not supported in OpenShift "
pkg=Inventory
```

Note how the error message doesn't contain the OpenShift version at the
end. It should, as the code is like this:

```go
return common.NewApiError(http.StatusBadRequest, errors.Errorf("%s %s", "dual-stack VIPs are not supported in OpenShift", cluster.OpenshiftVersion))
```

So we are assuming, at least in that code, that the `openshift_version`
column is not empty.

This patch changes adds a database constraint to ensure that that
`openshift_column` isn't null or empty. That doesn't address the root
cause (because we don't know it) but at least it should catch situations
where something tries to set that column empty.

Related: https://issues.redhat.com/browse/OCPBUGS-42059
Related: https://github.com/openshift/assisted-service/blob/e59df80246f418b62048d7252e4d79bf5723c81e/internal/cluster/validations/validations.go#L256
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>

swagger.yaml

@@ -5150,6 +5150,7 @@ definitions:
       openshift_version:
         type: string
         description: Version of the OpenShift cluster.
+        x-go-custom-tag: gorm:"not null;check:openshift_version <> ''"
       ocp_release_image:
         type: string
         description: OpenShift release image URI.