An example of how you can conditionally create resources in CloudFormation using Fn::If or !If for short.

fn-if-example.yaml

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  SnsTopicArn:
    Type: String
    Default: ''
    Description: >-
      A pre-existing SNS topic to send S3 notifications to. 
      If not provided, a new topic will be created.
  MyEmail:
    Type: String
    Description: The email address where you want to receive messages.
Conditions:
  SnsTopicNotProvided: !Equals [!Ref SnsTopicArn, '']
Resources:
  MyTopic:
    Condition: SnsTopicNotProvided
    Type: 'AWS::SNS::Topic'
  MySubscription:
    Type: AWS::SNS::Subscription
    Properties:
      Endpoint: !Ref MyEmail
      Protocol: email
      TopicArn: !If [SnsTopicNotProvided, !Ref MyTopic, !Ref SnsTopicArn]