[BUG] "build_only_models: true" causes missing "import 'dart:convert';"

[jl-insiders]

First I wanted to say thank you for sharing this library. Currently, we are using the models only feature which was working well up to v2.3.13.

Describe the bug
When setting "build_only_models: true" in build.yaml,
the generated file: pet_service_json.swagger.dart
is missing "import 'dart:convert';" for jsonEncode.

To Reproduce
I can reproduce this using the swagger file from the example in this repo.
Edit build.yaml and set "build_only_models" to true.
flutter pub run build_runner build --delete-conflicting-outputs
Then look in pet_service_json.swagger.dart and notice that
import 'dart:convert'; is not in the file.

Library version used:
v2.4.7

[Vovanella95]

Hi @jl-insiders will take a look into it asap