fix(serverless-openapi): handle optional response correctly

Author: Nightapes

packages/serverless-openapi/src/lib/generator.ts

@@ -212,11 +212,17 @@ export class Generator {
   }
 
   private handleResponses(
-    responseSchemas: { [key: string]: { 'application/json': Schema } },
+    responseSchemas:
+      | { [key: string]: { 'application/json': Schema } }
+      | undefined,
     openApi: OpenAPIV3.Document
-  ): OpenAPIV3.ResponsesObject {
+  ): OpenAPIV3.ResponsesObject | undefined {
     const responses: OpenAPIV3.ResponsesObject = {};
 
+    if (!responseSchemas) {
+      return undefined;
+    }
+
     for (const code of Object.keys(responseSchemas)) {
       const schemaJSON = responseSchemas[code]['application/json'];
       responses[code] = {