[BUG][typescript-fetch] Default case for oneOf serialization method returning undefined variable. (#21669)

* fix bug for returning uninitialized variable for serialization method in oneOf models with discriminator

* Update samples

---------

Co-authored-by: Chris Gual <cgual@omnidian.com>

modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache

@@ -142,7 +142,7 @@ export function {{classname}}ToJSONTyped(value?: {{classname}} | null, ignoreDis
             return Object.assign({}, {{modelName}}ToJSON(value), { {{discriminator.propertyName}}: '{{mappingName}}' } as const);
 {{/discriminator.mappedModels}}
         default:
-            return json;
+            return value;
     }
 {{/discriminator}}
 {{^discriminator}}

samples/client/petstore/typescript-fetch/builds/oneOf/models/TestDiscriminatorResponse.ts

@@ -66,7 +66,7 @@ export function TestDiscriminatorResponseToJSONTyped(value?: TestDiscriminatorRe
         case 'optionTwo':
             return Object.assign({}, OptionTwoToJSON(value), { discriminatorField: 'optionTwo' } as const);
         default:
-            return json;
+            return value;
     }
 }