Schema validation does not return error for invalid schema definition

[NV4RE]

When using the gojsonschema library, I encountered an issue where the schema validation does not return an error if the schema definition itself is not valid. This can lead to confusion and unexpected behavior, as the validation process proceeds without indicating that the schema is malformed.

https://go.dev/play/p/AlvO42OM74F

package main

import (
    "fmt"
    "github.com/xeipuuv/gojsonschema"
)

func main() {
    schemaLoader := gojsonschema.NewStringLoader(`{"i am not": "real schema"}`)
    documentLoader := gojsonschema.NewStringLoader(`{"snake_snake": 3,"foo":{"bar":"baz","animals":["dog","cat","pengiun"],"number":3}}`)

    result, err := gojsonschema.Validate(schemaLoader, documentLoader)
    if err != nil {
        panic(err.Error())
    }

    if result.Valid() {
        fmt.Printf("The document is valid\n")
    } else {
        fmt.Printf("The document is not valid. see errors :\n")
        for _, desc := range result.Errors() {
            fmt.Printf("- %s\n", desc)
        }
    }
}

Expected Behavior
The program should return an error indicating that the schema definition is not valid.

Actual Behavior
The program does not return an error for the invalid schema definition and proceeds to validate the document against the malformed schema.

Environment
gojsonschema version: v1.2.0
Go version: go1.21.5
OS: darwin/arm64

[siller174]

It seems that we can only check by the "type" in the root schema. If it is present, we should return an error.

https://github.com/xeipuuv/gojsonschema/pull/379/files