System.NullReferenceException in AzureStorageOrchestrationService.CompleteTaskOrchestrationWorkItemAsync

Version: Core 2.4.1, AzureStorate: 1.8.1

For our use case we use sub orchestrations and external events, however we observed random NPE and our main orchestration instance get stuck since the sub orchestration instance never gets created.

Exception:
```
DurableTask.Core[29]
TaskOrchestrationDispatcher-1bc6efed64814fb897c9e58d6f22c01b-0: Unhandled exception with work item 'fb874f1e8645614ea67e2330b4c60fcd': System.NullReferenceException: Object reference not set to an instance of an object.
at DurableTask.AzureStorage.AzureStorageOrchestrationService.CompleteTaskOrchestrationWorkItemAsync(TaskOrchestrationWorkItem workItem, OrchestrationRuntimeState newOrchestrationRuntimeState, IList`1 outboundMessages, IList`1 orchestratorMessages, IList`1 timerMessages, TaskMessage continuedAsNewMessage, OrchestrationState orchestrationState) in C:\source\durabletask\src\DurableTask.AzureStorage\AzureStorageOrchestrationService.cs:line 1005
at DurableTask.Core.TaskOrchestrationDispatcher.OnProcessWorkItemAsync(TaskOrchestrationWorkItem workItem) in C:\source\durabletask\src\DurableTask.Core\TaskOrchestrationDispatcher.cs:line 463
at DurableTask.Core.TaskOrchestrationDispatcher.OnProcessWorkItemSessionAsync(TaskOrchestrationWorkItem workItem) in C:\source\durabletask\src\DurableTask.Core\TaskOrchestrationDispatcher.cs:line 194
at DurableTask.Core.WorkItemDispatcher`1.ProcessWorkItemAsync(WorkItemDispatcherContext context, Object workItemObj) in C:\source\durabletask\src\DurableTask.Core\WorkItemDispatcher.cs:line 459

Backing off for 10 seconds until 5 successful operations
```

While debugging locally we found this line gets `runtimeState.OrchestrationInstance` as null.
https://github.com/Azure/durabletask/blob/bcc97e874baba09c722a83c164bb3ab2245017ec/src/DurableTask.AzureStorage/AzureStorageOrchestrationService.cs#L948

below logic in the same method assumes the `runtimeState.OrchestrationInstance` can be null but not that line.
example line that expects nullable instance
https://github.com/Azure/durabletask/blob/bcc97e874baba09c722a83c164bb3ab2245017ec/src/DurableTask.AzureStorage/AzureStorageOrchestrationService.cs#L972