[BUG][typescript-nestjs] Generated NestJS Module does not compile with "strict" flag

[swcm-mnestler]

Description

Clients generated with the typescript-nestjs experimental generator will not be compilable with the strict typescript compilation flag due to nullability issues in the generated ApiModule files.

openapi-generator version

Occurs in 7.2.0, used to work in 6.6.0. However, even in 6.6.0, certain features (which we don't use) also cause this to occur - see reproducer.

OpenAPI declaration file content or url

For the reproducer, I used the petstore project

Generation Details

npx @openapitools/openapi-generator-cli generate -g typescript-nestjs -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml

Steps to reproduce

• Generate client using command above
• Setup rest of dependencies (npm i typescript axios @nestjs/common @nestjs/axios, npx tsc init)
• Setup necessary compilation flags in tsconfig.json ("experimentalDecorators": true, "emitDecoratorMetadata": true)
• Run typecheck (npx tsc --noEmit)

A ready-to-use repository with the steps above taken is available at https://github.com/swcm-mnestler/nestjs-openapi-reproducer

Suggest a fix

I'll create a MR for the nullability instances that I encountered, but it's possible there are more of these occurrences in the code base.