fix NullReferenceException in task hub worker shutdown

[sebastianburckhardt]

I noticed that the task hub worker does not shut down correctly if some functions are disabled with the [Disable] attribute.

The reason is that the info variable can be null in the following checks that happen during shutdown:

 if (this.isTaskHubWorkerStarted &&
                    !this.knownOrchestrators.Values.Any(info => info.HasActiveListener) &&
                    !this.knownActivities.Values.Any(info => info.HasActiveListener) &&
                    !this.knownEntities.Values.Any(info => info.HasActiveListener))

As far as I can see this is easily fixed by ignoring null entries for the purpose of this check.

[cgillum]

Oh that's interesting. I thought we had already covered the case where functions where disabled. At least, that's what I had in this comment here:

azure-functions-durable-extension/src/WebJobs.Extensions.DurableTask/Bindings/RegisteredFunctionInfo.cs

Lines 18 to 19 in f1a4291

	// This flag is set when a function is disabled or the host is shutting down.
	internal bool IsDeregistered { get; set; }

So either I tested this wrong originally, or something changed, but it's very clear from the code that nulls can exist here. Thanks for submitting this fix! Maybe you can add a comment above your change explaining that null is expected for disabled functions?

[cgillum]

I like your local function approach. Very clean and easy to understand. :) LGTM as long as the CI is green.