Make StringData property nullable in WorkflowDefinition

Updated the `StringData` property to allow null values, improving flexibility and aligning with usage scenarios where the property may not always contain a value. This change ensures better handling of optional data.

src/modules/Elsa.Workflows.Management/Entities/WorkflowDefinition.cs

@@ -77,7 +77,7 @@ public class WorkflowDefinition : VersionedEntity
     /// <summary>
     /// A textual representation of the workflow. The data is to be interpreted by the configured materializer.
     /// </summary>
-    public string StringData { get; set; } = null!;
+    public string? StringData { get; set; }
 
     /// <summary>
     /// A binary representation of the workflow. The data is to be interpreted by the configured materializer.