getPopulatableFields fails when cloning schemae

[hennessyevan]

Describe the bug
In mongoose, cloning models produces some internal fields that are picked up by mode.schema.eachPath() that have no types which causes an error on Line 183 of registry.ts and other lines following where type is expected to be defined but in fact is undefined.

To Reproduce
Create a schema and a clone it for that model as in the example in the Mongoose Docs

Expected behavior
Mogr should ignore such typeless fields as they are only for internal mongoose/(user?) logic and not projectable anyways.

Desktop (please complete the following information):

• OS: macOS 10.15
• NodeJS Version: 10.16.3
• GraphQL Version: 14.5.8
• Mongoose Version: 5.7.5

I imagine just adding a simple

if (!type) {
  return []
}

before Line 183 of registry.ts should do the trick