Models Validation doesn't check the implicitly created AWS::Serverless::Api resource

### Description:
The Models Validation doesn't check the implicitly created `AWS::Serverless::Api` resource. When creating a new model there's no way to set it to the implicitly `AWS::Serverless::Api resource` without `sam validate` failing. Models isn't a valid global parameter so it can't be set there. Creating a AWS::ApiGateway::Model resource setting RestApiId to `ServerlessRestApi` the following validation error is thrown `...the related API does not define any Models`. This means the only way to add a Model is to explicitly create the Api resource, add the Models to it, and then set RestApiId on the other resources. This is a pain point for us because we have 75 Function resources in the template that now need to explicitly set RestApiId.

### Steps to reproduce:
```
Account:
Type: 'AWS::ApiGateway::Model'
Properties:
RestApiId: !Ref ServerlessRestApi
ContentType: application/json
Name: Account
Schema:
$schema: 'http://json-schema.org/draft-04/schema#'
title: Account
type: array
items:
type: object
properties:
name:
type: string
status:
type: string
```

```
CreateAccountFunction:
Type: AWS::Serverless::Function
Properties:
Handler: controllers/accounts.create
Events:
CreateAccount:
Type: Api
Properties:
Path: /accounts
Method: post
RequestModel:
Model: Account
Required: true
ValidateBody: true
Role: !GetAtt LambdaRole.Arn
```

### Observed result:
`Error: [InvalidResourceException('CreateAccountFunction', 'Event with id [CreateAccount] is invalid. Unable to set RequestModel [CreateAccountsModel] on API method [post] for path [/accounts] because the related API does not define any Models.')] ('CreateAccountFunction', 'Event with id [CreateAccount] is invalid. Unable to set RequestModel [CreateAccountsModel] on API method [post] for path [/accounts] because the related API does not define any Models.')`

### Expected result:
I would expect the validation to check if any models exist on the implicitly created Api resource.