JSInterop: Enable returning null for a nullable value type

When a result value is null, and the return type is a nullable value type, Convert.ChangeType throws instead of returning the null value.

dotnet#30366

Author: Regenhardt

src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs

@@ -89,7 +89,9 @@ public void SetResult(object tcs, object? result)
             // If necessary, attempt a cast
             var typedResult = result is T resultT
                 ? resultT
-                : (T)Convert.ChangeType(result, typeof(T), CultureInfo.InvariantCulture)!;
+                : result == null && default(T) == null
+                    ? default(T)
+                    : (T)Convert.ChangeType(result, typeof(T), CultureInfo.InvariantCulture)!;
 
             typedTcs.SetResult(typedResult!);
         }