Fix parameter's default value

Description

Currently the parameter's default value is obtained via the following code

            if (parameterSchema.getDefault() != null) {
                codegenParameter.defaultValue = toDefaultValue(parameterSchema);
            }

We need to revise it to skip the null check in the default codegen and let the generator to handle it (with null value) instead.

We will need to revise all generators as the change impacts default codegen.

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/1027 #761

Suggest a fix/enhancement

I'm working on a fix, which is due by coming Sunday.