Query params should not be nullable

[sergiimk]

Consider a query params struct like this:

#[derive(Debug, serde::Deserialize, utoipa::IntoParams)]
#[serde(rename_all = "camelCase", deny_unknown_fields)]
#[into_params(parameter_in = Query)]
pub struct QueryParams {
    pub schema_format: Option<SchemaFormat>,
}

#[derive(
    Debug,
    Default,
    Clone,
    Copy,
    PartialEq,
    Eq,
    serde::Serialize,
    serde::Deserialize,
    utoipa::ToSchema,
)]
pub enum SchemaFormat {
    #[default]
    ArrowJson,
    Parquet,
    ParquetJson,
}

Utoipa generates a spec like this:

{
  "openapi": "3.1.0",
  "paths": {
    "/query": {
      "get": {
        "operationId": "query_handler",
        "parameters": [
          {
            "description": "How to encode the schema of the result",
            "in": "query",
            "name": "schemaFormat",
            "required": false,
            "schema": {
              "oneOf": [
                {
                  "type": "null"
                },
                {
                  "$ref": "#/components/schemas/SchemaFormat"
                }
              ]
            }
          }
       ]
      }
    }
  },
  "components": {
    "schemas": {
      "SchemaFormat": {
        "enum": [
          "ArrowJson",
          "Parquet",
          "ParquetJson"
        ],
        "type": "string"
      }
    }
  }
}

When we run IBM openapi-validator on this spec it gives an error:

Errors:

  Message :   Invalid type; valid types are: array, boolean, integer, number, object, string
  Rule    :   ibm-schema-type-format
  Path    :   paths./query.get.parameters.5.schema.oneOf.0

I think the validator is correct here, as in case of a query parameter there cannot be null value, as it would be non-distinguishable from a "null" string.

My expectation is that for an optional query parameter utoipa should only specify "required": false but not make value nullable.