fix(HasOneAttribute): return null if property doesnt exist

Author: jaredcnance

src/JsonApiDotNetCore/Models/HasOneAttribute.cs

@@ -54,9 +54,20 @@ public override void SetValue(object entity, object newValue)
 
         // HACK: this will likely require boxing
         // we should be able to move some of the reflection into the ContextGraphBuilder
+        /// <summary>
+        /// Gets the value of the independent identifier (e.g. Article.AuthorId)
+        /// </summary>
+        /// 
+        /// <param name="entity">
+        /// An instance of dependent resource
+        /// </param>
+        /// 
+        /// <returns>
+        /// The property value or null if the property does not exist on the model.
+        /// </returns>
         internal object GetIdentifiablePropertyValue(object entity) => entity
                 .GetType()
                 .GetProperty(IdentifiablePropertyName)
-                .GetValue(entity);
+                ?.GetValue(entity);
     }
 }