Update sub-orchestration default versioning

src/Abstractions/TaskOptions.cs

@@ -94,7 +94,7 @@ public SubOrchestrationOptions(TaskOptions options, string? instanceId = null)
     /// <summary>
     /// Gets the version to associate with the sub-orchestration instance.
     /// </summary>
-    public TaskVersion Version { get; init; } = default!;
+    public TaskVersion? Version { get; init; }
 }
 
 /// <summary>

src/Client/Grpc/GrpcDurableTaskClient.cs

@@ -79,7 +79,7 @@
         Check.NotEntity(this.options.EnableEntitySupport, options?.InstanceId);
 
         // We're explicitly OK with an empty version from the options as that had to be explicitly set. It should take precedence over the default.
-        string version = string.Empty;
+        string? version = null;
         if (options?.Version is { } v)
         {
             version = v;
@@ -108,7 +108,7 @@
 
         if (Activity.Current?.Id != null || Activity.Current?.TraceStateString != null)
         {
             if (request.ParentTraceContext == null)
             {
                 request.ParentTraceContext = new P.TraceContext();
             }

src/Worker/Core/Shims/TaskOrchestrationContextWrapper.cs

@@ -179,9 +179,8 @@ public override async Task<TResult> CallSubOrchestratorAsync<TResult>(
         static string? GetInstanceId(TaskOptions? options)
             => options is SubOrchestrationOptions derived ? derived.InstanceId : null;
         string instanceId = GetInstanceId(options) ?? this.NewGuid().ToString("N");
-        string defaultVersion = this.invocationContext.Options?.Versioning?.DefaultVersion ?? string.Empty;
-        string version = options is SubOrchestrationOptions subOptions ? subOptions.Version : defaultVersion;
-
+        string defaultVersion = this.GetDefaultVersion();
+        string version = options is SubOrchestrationOptions { Version: { } v } ? v.Version : defaultVersion;
         Check.NotEntity(this.invocationContext.Options.EnableEntitySupport, instanceId);
 
         // if this orchestration uses entities, first validate that the suborchestration call is allowed in the current context
@@ -482,4 +481,22 @@ async Task<T> InvokeWithCustomRetryHandler<T>(
             }
         }
     }
+
+    // The default version can come from two different places depending on the context of the invocation.
+    string GetDefaultVersion()
+    {
+        // Preferred choice.
+        if (this.invocationContext.Options.Versioning?.DefaultVersion is { } v)
+        {
+            return v;
+        }
+
+        // Secondary choice.
+        if (this.Properties.TryGetValue("defaultVersion", out var propVersion) && propVersion is string v2)
+        {
+            return v2;
+        }
+
+        return string.Empty;
+    }
 }