[Internal]: Fixes NullRef in TransportHandler when client is concurrently disposed. (#5062)

# Pull Request Template

## Description

Fixes a race conditions where StoreProxy returned can be null when the
client is getting disposed at the same time.

NOTE: Testing this is not easy due to this ebing a race condition. I
evaluated using the ExceptionLess tests (which is using
MockTransportHandler) - but it was not sutainable - because the race
condition to hit the NullRef is tiny. Given that throwing
ObjectDisposedException happens in other places regularly in this code
path I think it is acceptable to not add a unit tests trying to provoke
this edge condition.

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [] New feature (non-breaking change which adds functionality)
- [] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [] This change requires a documentation update

## Closing issues

To automatically close an issue: closes #IssueNumber

Author: FabianMeiswinkel

Microsoft.Azure.Cosmos/src/Handler/TransportHandler.cs

@@ -108,6 +108,19 @@ internal async Task<ResponseMessage> ProcessMessageAsync(
             serviceRequest.Headers[HttpConstants.HttpHeaders.Authorization] = authorization;
 
             IStoreModel storeProxy = this.client.DocumentClient.GetStoreProxy(serviceRequest);
+            if (storeProxy == null)
+            {
+                // storeProxy being null should indicate that there was a race condition and the Client was
+                // disposed between getting teh DocumentClient and calling GetStoreProxy
+                // in this case repeating this call will result in throwing an ObjectDisposedException
+                storeProxy = this.client.DocumentClient.GetStoreProxy(serviceRequest);
+
+                if (storeProxy == null)
+                {
+                    throw new InvalidOperationException("StoreProxy cannot be null");
+                }
+            }
+
             using (ITrace processMessageAsyncTrace = request.Trace.StartChild(
                             name: $"{storeProxy.GetType().FullName} Transport Request",
                             component: TraceComponent.Transport,