Optional (nullable) enums fail JSON validation when explicitly set to "null"

[dmlambea]

Describe the bug
A not required, nullable enum field fails JSON validation whenever is sent with explicit "null" value. If not sent at all, it passes validation.

To Reproduce
Run a server with: https://go.dev/play/p/0JQGk_sT-Ox?v=goprev
Then, try the code using cURL as specified below.

Expected behavior
Sending a request with the enum field set to null should validate correctly.

Additional context
My go.mod is:

module rest_example

go 1.23.1

require (
        github.com/go-chi/chi/v5 v5.2.3
        github.com/swaggest/rest v0.2.75
        github.com/swaggest/usecase v1.3.1
)

require (
        github.com/cespare/xxhash/v2 v2.3.0 // indirect
        github.com/santhosh-tekuri/jsonschema/v3 v3.1.0 // indirect
        github.com/swaggest/form/v5 v5.1.1 // indirect
        github.com/swaggest/jsonschema-go v0.3.78 // indirect
        github.com/swaggest/openapi-go v0.2.59 // indirect
        github.com/swaggest/refl v1.4.0 // indirect
        gopkg.in/yaml.v2 v2.4.0 // indirect
)

Test the code using cURL, with the field "enum" intentionally set to null:

curl -v 'http://localhost:8080/example' \
   -H 'accept: application/json' \
   -H 'Content-Type: application/json' \
   -d '{ "name": "Example name", "desc": "A description", "enum": null }'

[dmlambea]

I traced the problem down to the line 203 of the schema.go file of the santhosh-tekuri/jsonschema v3.1.0 dependency.

The issue seems to be that it checks whether the received value is within the possible options of the enum, but it does not take into account that the value is nil and that field is defined as nullable:

  ...

  if len(s.Enum) > 0 {      //  <-- this line not checking the nil value
    matched := false
    for _, item := range s.Enum {
      if equals(v, item) {
        matched = true
        break
      }
    }
    if !matched {
      errors = append(errors, validationError("enum", s.enumError))
    }
  }

  ...

[dmlambea]

Hello!
Is there any progress with this?