Fixes + unit tests for streaming PubSub implementation

[WhitWaldo]

Description

Fixed the linked issue which called out the following problems:

• Sometimes inbound messages from the Dapr runtime are null which cause a NullReferenceError during delegate processing. This has been updated to add a null check and skip such messages.
• The implementation used an await Task.WhenAny which caused the whole of the operation to be blocking. This was unintentional and has been corrected.

While fixing these, I realized that the project was devoid of several useful unit tests that could ensure that the rest of the class was operating as it should, so I took the opportunity to add a few internal fields and methods (available only to the test project) that would assist in some of this validation.

Finally, I modified the AddDaprPubSubClient extension method to pass an IConfiguration? into the DaprPublishSubscribeClientBuilder so it can optionally source the various configuration-based values from there during registration.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #1412

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• [N/A] Extended the documentation - done in another issue