Dashboard not crash if a scheduling task is cancelled

If a task is cancelled with a IClientFilter, then LastJobId is empty. The dashboard cannot manage this behavior
Please use the `Moq` library to build mocks instead of fakes (there are a lot of other examples in the test project):

```csharp
var connection = new Mock<IStorageConnection>();
connection.Setup(x => x.GetAllEntriesFromHash("the-key"))
    .Returns(new Dictionary<string, string>
    {
        /* ... */
    });
```