Add tests for nullable type

We have spun out a separate package, `oapi-codegen/nullable` as a step
towards oapi-codegen/oapi-codegen#1039.

Until we have implemented #27, we cannot add an explicit type alias in
this package, so we can at least add some tests to cover additional
functionality and expectations that the package should have when
interplaying with `oapi-codegen`.

Co-authored-by: Sebastien Guilloux <[email protected]>
Co-authored-by: Ashutosh Kumar <[email protected]>

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/google/uuid v1.5.0
 	github.com/kataras/iris/v12 v12.2.6-0.20230908161203-24ba4e8933b9
 	github.com/labstack/echo/v4 v4.11.4
+	github.com/oapi-codegen/nullable v1.0.0
 	github.com/stretchr/testify v1.8.4
 )
 

go.sum

@@ -122,6 +122,8 @@ github.com/modern-go/reflect2 v1.0.2 h1:xBagoLtFs94CBntxluKeaWgTMpvLxC4ur3nMaC9G
 github.com/modern-go/reflect2 v1.0.2/go.mod h1:yWuevngMOJpCy52FWWMvUC8ws7m/LJsjYzDa0/r8luk=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e h1:fD57ERR4JtEqsWbfPhv4DMiApHyliiK5xCTNVSPiaAs=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
+github.com/oapi-codegen/nullable v1.0.0 h1:FjV9h/GLxYxc3wSSFUafVMvi0lhrpvUcCbEkE04y0fw=
+github.com/oapi-codegen/nullable v1.0.0/go.mod h1:KUZ3vUzkmEKY90ksAmit2+5juDIhIZhfDl+0PwOQlFY=
 github.com/pelletier/go-toml/v2 v2.0.9 h1:uH2qQXheeefCCkuBBSLi7jCiSmj3VRh2+Goq2N7Xxu0=
 github.com/pelletier/go-toml/v2 v2.0.9/go.mod h1:tJU2Z3ZkXwnxa4DPO899bsyIoywizdUvyaeZurnPPDc=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=