Datamodel types without scalar fields break GQLify generation

[alexandrethsilva]

Describe the bug When creating a data model type without any scalar fields (ie. only relation fields), code generation breaks.

To Reproduce Steps to reproduce the behavior:

1. Create a data model type without scalar fields, eg:

type Foo {
  bar: Bar
}

2. Run generator
3. See error

return new _GraphQLError.GraphQLError(
       ^
Syntax Error: Expected Name, found }

GraphQL request (1:37)
1: input UserSubscriptionsCreateInput {}

Expected behavior No error when running the generation.

Desktop

• OS: OS X
• OS version: 10.14.3 Beta (18D32a)
• GQLify package versions: @gqlify/firestore: 3.1.0, @gqlify/server: 3.1.2,

Additional context This is exactly the same issue that is happening with Prisma on this issue, so I'm kinda copy/pasting it here.