title: Input Object type `TypeName` must define one or more fields
body: <p>I am experimenting with <code>NestJS</code> and <code>TypeGraphQL</code>. And I have an example model of a cat.</p>
  
  <pre><code>import { Document } from 'mongoose';
  import { ObjectType, InputType, Field, ID } from 'type-graphql';
  
  export interface Cat extends Document {
    readonly name: string;
    readonly age?: number;
  }
  
  class CatBase {
    @Field()
    name: string;
  
    @Field({ nullable: true })
    age?: number;
  }
  
  @ObjectType()
  export class CatObjectType extends CatBase implements Cat {
    @Field(type =&gt; ID)
    id: string;
  }
  
  @InputType()
  export class CatInputType extends CatBase implements Cat {
  }
  </code></pre>
  
  <p>Here I am trying to reuse <code>BaseCat</code> in <code>CatObjectType</code> and <code>CatInputType</code>. But I getting this error:</p>
  
  <pre><code>[ { GraphQLError: Input Object type CatInputType must define one or more fields.
        at SchemaValidationContext.reportError (/Users/pavel/Dev/exampleproject/node_modules/graphql/type/validate.js:90:19)
        at validateInputFields (/Users/pavel/Dev/exampleproject/node_modules/graphql/type/validate.js:432:13)
        at validateTypes (/Users/pavel/Dev/exampleproject/node_modules/graphql/type/validate.js:240:7)
        at validateSchema (/Users/pavel/Dev/exampleproject/node_modules/graphql/type/validate.js:54:3)
        at graphqlImpl (/Users/pavel/Dev/exampleproject/node_modules/graphql/graphql.js:79:62)
        at /Users/pavel/Dev/exampleproject/node_modules/graphql/graphql.js:28:59
        at new Promise (&lt;anonymous&gt;)
        at Object.graphql (/Users/pavel/Dev/exampleproject/node_modules/graphql/graphql.js:26:10)
        at Function.&lt;anonymous&gt; (/Users/pavel/Dev/exampleproject/node_modules/type-graphql/dist/schema/schema-generator.js:18:52)
        at Generator.next (&lt;anonymous&gt;)
        at /Users/pavel/Dev/exampleproject/node_modules/tslib/tslib.js:110:75
        at new Promise (&lt;anonymous&gt;)
        at Object.__awaiter (/Users/pavel/Dev/exampleproject/node_modules/tslib/tslib.js:106:16)
        at Function.generateFromMetadata (/Users/pavel/Dev/exampleproject/node_modules/type-graphql/dist/schema/schema-generator.js:15:24)
        at /Users/pavel/Dev/exampleproject/node_modules/type-graphql/dist/utils/buildSchema.js:11:65
        at Generator.next (&lt;anonymous&gt;)
      message:
       'Input Object type CatInputType must define one or more fields.' } ]
  (node:72485) UnhandledPromiseRejectionWarning: Error: Generating schema error
      at Function.&lt;anonymous&gt; (/Users/pavel/Dev/exampleproject/node_modules/type-graphql/dist/schema/schema-generator.js:20:27)
      at Generator.next (&lt;anonymous&gt;)
      at fulfilled (/Users/pavel/Dev/exampleproject/node_modules/tslib/tslib.js:107:62)
      at processTicksAndRejections (internal/process/next_tick.js:81:5)
  (node:72485) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
  (node:72485) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
  </code></pre>
  
  <p>When in <code>CatInputType</code> are described all fields from <code>BaseCat</code> all work as expected. What I am doing wrong?</p>