Is your feature request related to a problem? Please describe.

When using akka-http-server generator, non required field of model has no default value.

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/scala-akka-http-server/model.mustache#L22

This makes it necessary to explicitly provide the values of the added non-required fields when they are added to the generated model at the user of the model.

On the other hand, akka-client generator provides the default value, and doesn’t have the same issue.
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/scala-akka-client/model.mustache#L16

Describe the solution you'd like

Add None as default value when the field is required, in imitation of akka-client

Describe alternatives you've considered

Additional context