fix: ApiKeyRequired is not working as expected in api event when there is no `Auth` section for the Api

[ShreyaGangishetty]

Description:
SAM throws a 5xx exception when there is no Auth section for API and ApiKeyRequired is set at the function level.

Steps to reproduce the issue:

1. Deploy the below template using SAM

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Resources:
  MyFunctionWithApiKeyRequired:
    Type: AWS::Serverless::Function
    Properties:
      InlineCode: |
         exports.handler = async (event) => {
          console.log("Hello from MyAuthFunction")
           return {
           statusCode: 200,
           body: JSON.stringify(event),
           headers: {}
           }
         }
      Handler: index.handler
      Runtime: nodejs8.10
      Events:
        MyApiWithApiKeyRequired:
          Type: Api
          Properties:
            Path: /hello
            Method: get
            Auth:
              ApiKeyRequired: false

Observed result:

Traceback (most recent call last):
  File "/var/task/handlers.py", line 67, in transform
    output_fragment = translator.translate(copy.deepcopy(input_fragment), parameter_values=parameter_values)
  File "/var/task/samtranslator/translator/translator.py", line 83, in translate
    translated = macro.to_cloudformation(**kwargs)
  File "/var/task/samtranslator/model/sam_resources.py", line 133, in to_cloudformation
    lambda_alias=lambda_alias)
  File "/var/task/samtranslator/model/sam_resources.py", line 357, in _generate_event_resources
    resources += eventsource.to_cloudformation(**kwargs)
  File "/var/task/samtranslator/model/eventsources/push.py", line 534, in to_cloudformation
    self._add_swagger_integration(explicit_api, function)
  File "/var/task/samtranslator/model/eventsources/push.py", line 635, in _add_swagger_integration
    if apikey_required_setting_is_false and not api_auth.get('ApiKeyRequired'):
AttributeError: 'NoneType' object has no attribute 'get'

Expected result:
throw an ERROR for ApiKeyRequired:false if there is no ApiKeyRequired:true set for the api and set the required auth in security for the function path if ApiKeyRequired:true at function level and not present at api level

[keetonian]

Released!