Add null check for _jobServerQueue

[max-zaytsev]

Added null check for the job server queue in the UpadteMetadata method.

In rare cases, the agent receives a Metadata update message from the server when the job completes and the _jobServerQueue has already been set to null. So the user can see the null refference exception in the pipeline logs:

In the worker logs we can see the following:

[2022-01-19 00:54:54Z INFO JobRunner] Shutting down the job server queue.
...
[2022-01-19 00:55:08Z INFO JobServerQueue] All queue process tasks have been stopped, and all queues are drained.
...
[2022-01-19 00:55:08Z INFO JobRunner] Raising job completed event.
[2022-01-19 00:55:08Z INFO Worker] Metadata update message received.
[2022-01-19 00:55:08Z ERR  Program] System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.VisualStudio.Services.Agent.Worker.JobRunner.UpdateMetadata(JobMetadataMessage message)
   at Microsoft.VisualStudio.Services.Agent.Worker.Worker.RunAsync(String pipeIn, String pipeOut)
   at Microsoft.VisualStudio.Services.Agent.Worker.Program.MainAsync(IHostContext context, String[] args)

This PR should fix this issue

---

Also added 2 L0 tests:

• DontUpdateWebConsoleLineRateIfJobServerQueueIsNull
• UpdateWebConsoleLineRateIfJobServerQueueIsNotNull

Test results:

.\dev.cmd l0

Passed!  - Failed:     0, Passed:   765, Skipped:     0, Total:   765, Duration: 13 s - Test.dll (netcoreapp3.1)

[anatolybolshakov]

@max-zaytsev could you please cover it by L0 tests also?

[anatolybolshakov]

@max-zaytsev please share manual test results as well