Fix returning NoContent. (#9830)

Author: mgajda

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellServantCodegen.java

@@ -564,7 +564,7 @@ public CodegenOperation fromOperation(String resourcePath, String httpMethod, Op
         // Add the HTTP method and return type
         String returnType = op.returnType;
         if (returnType == null || returnType.equals("null")) {
-            returnType = "()";
+            returnType = "NoContent";
         }
         if (returnType.indexOf(" ") >= 0) {
             returnType = "(" + returnType + ")";