Providing an unknown field as if it were a list results in cryptic error

When you provide an unknown field as if it were a list, rather than getting an error stating the field does not exist, you get the following error:

Argument 1 passed to GraphQL\Type\Definition\Type::getNullableType() must be an instance of GraphQL\Type\Definition\Type, null given, called in /src/vendor/webonyx/graphql-php/src/Validator/Rules/ValuesOfCorrectType.php on line 75

For example, I have this input type:

input SomeInput {
    name: String!
}

And in the query you add an unknown field like this:

{
  name: "Example"
  unknown: [
  ]
}

You'll get the error mentioned above.

However, if you add the unknown field like this:

{
  name: "Example"
  unknown: "Foo"
}

You simply get an error message stating "Field "unknown" is not defined by type SomeInput."

I would expect in both cases to get the error "Field "unknown" is not defined by type SomeInput.".

Steps to reproduce

1. Create any input type
2. Provide an extra field that is not present in the input type, and make it's value a list
3. View error

v5.6.0

Reproduce the issue with an input type containing name and an unknown list-valued field, then inspect the validation path at vendor/webonyx/graphql-php/src/Validator/Rules/ValuesOfCorrectType.php around line 75. Compare it with the unknown scalar-field case. Done means both forms report that the field is not defined by type SomeInput.