fix: throw proper errors when `Authorizer: NONE` is set in api event and there are no authorizers defined in api

[ShreyaGangishetty]

Description:
If there is no Authorizer for AWS::Serverless::Api resource and you set Authorizer: NONE at api event level sam doesn't throw proper errors.

Sample Template:

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      InlineCode: |
        def handler (event, context):
          print("hello") 
      Handler: index.handler
      Runtime: python3.7
      Events:
        HttpGetUserGroupIdUserId:
          Type: Api
          Properties:
            Path: '/users/{groupId}/{userId}'
            Method: GET
            Auth:
              Authorizer: "NONE"

Observed result:

Traceback (most recent call last):
  File "bin/sam-translate.py", line 141, in <module>
    transform_template(input_file_path, output_file_path)
  File "bin/sam-translate.py", line 98, in transform_template
    sam_template, {}, ManagedPolicyLoader(iam_client))
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/translator/transform.py", line 16, in transform
    return translator.translate(input_fragment, parameter_values=parameter_values)
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/translator/translator.py", line 120, in translate
    translated = macro.to_cloudformation(**kwargs)
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/model/sam_resources.py", line 125, in to_cloudformation
    lambda_alias=lambda_alias)
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/model/sam_resources.py", line 335, in _generate_event_resources
    resources += eventsource.to_cloudformation(**kwargs)
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/model/eventsources/push.py", line 534, in to_cloudformation
    self._add_swagger_integration(explicit_api, function)
  File "/Users/shreygan/git/serverless-application-model/bin/../samtranslator/model/eventsources/push.py", line 626, in _add_swagger_integration
    if method_authorizer == 'NONE' and not api_auth.get('DefaultAuthorizer'):
AttributeError: 'NoneType' object has no attribute 'get'

Expected result:
Throw a proper error saying Auth cannot be set to NONE in Api Event when there is no DefaultAuthorizer/Authorizer at Api level