fix: Removes null serialization check.

This was preventing custom Scalars from representing items using Map.null. In the case where the value cannot be serialized, the serialize method itself should throw an error (not return null).

Author: NeedleInAJayStack

Sources/GraphQL/Execution/Execute.swift

@@ -1018,16 +1018,9 @@ func completeLeafValue(returnType: GraphQLLeafType, result: Any?) throws -> Map
     guard let result = result else {
         return .null
     }
-
     let serializedResult = try returnType.serialize(value: result)
 
-    if serializedResult == .null {
-        throw GraphQLError(
-            message:
-            "Expected a value of type \"\(returnType)\" but " +
-                "received: \(result)"
-        )
-    }
+    // Do not check for serialization to null here. Some scalars may model literals as `Map.null`.
 
     return serializedResult
 }