Using conditional "Fn::Equals" for cloudformation resources. From https://github.com/KlickMarketing/demo-site/blob/master/serverless.yml#L55 including no value

conditional-resources.yml

resources:
  Description: Demo Site
  Conditions:
    isProd: {"Fn::Equals" : ["${self:custom.stageFlag}", "prod"]}
    isRC: {"Fn::Equals" : ["${self:custom.stageFlag}", "rc"]}
    isDev: {"Fn::Equals" : ["${self:custom.stageFlag}", "dev"]}
    isProdOrRC: {"Fn::Or": [{"Condition": "isProd"}, {"Condition": "isRC" }]}
  Resources:
    ProxyEntry:
      Type: 'Custom::MarketingStackProxyEntry'
      Condition: isDev
      Version: '1.0'
      Properties:
        ServiceToken:
          Fn::ImportValue:
            Fn::Sub: "demo-stack-entry-cfn-dev-arn"
        Host:  ${self:custom.domain}
        Origin: {"Fn::GetAtt": [WebsiteBucket, DomainName]}
    WebsiteBucket:
      Type: "AWS::S3::Bucket"
      Properties:
        AccessControl: Private
        # AccelerateConfiguration:
    BucketPolicy:
      Type: "AWS::S3::BucketPolicy"
      Properties:
        Bucket: {Ref: WebsiteBucket}
        PolicyDocument:
          Version: "2012-10-17"
          Statement:
            Effect: Allow
            Principal:
              Fn::If:
                - isProdOrRC
                - CanonicalUser:
                  - {"Fn::GetAtt": [CloudfrontOAI, S3CanonicalUserId]}
                - CanonicalUser:
                  - {"Fn::ImportValue": "demo-stack-proxy-dev-S3CanonicalUserId"}
            Action:
              - s3:ListBucket
              - s3:GetObject
            Resource:
              - {"Fn::Join": ["", ["arn:aws:s3:::", {Ref: WebsiteBucket}]]}
              - {"Fn::Join": ["", ["arn:aws:s3:::", {Ref: WebsiteBucket}, "/*"]]}
    CloudfrontOAI:
      Type: "AWS::CloudFront::CloudFrontOriginAccessIdentity"
      Condition: isProdOrRC
      Properties:
        CloudFrontOriginAccessIdentityConfig:
          Comment: Created for ${self:custom.stackname}
    CDN:
      Type: "AWS::CloudFront::Distribution"
      Condition: isProdOrRC
      Properties:
        DistributionConfig:
          # WebACLId: 
          #   Fn::If:
          #     - isProd
          #     - {Ref: AWS::NoValue}
          #     - {Fn::ImportValue: "wafacl-prod"}
          Aliases:
            - ${self:custom.domain}
          Enabled: true
          HttpVersion: http2
          PriceClass: PriceClass_All
          IPV6Enabled: true
          DefaultRootObject: index.html
          CustomErrorResponses:
            - ErrorCode: 404
              ResponseCode: 404
              ResponsePagePath: /error.html
          Origins:
            - DomainName: {"Fn::GetAtt": [WebsiteBucket, DomainName]}
              Id: s3-origin
              S3OriginConfig:
                OriginAccessIdentity: {"Fn::Join": ["", ["origin-access-identity/cloudfront/", {Ref: CloudfrontOAI}]]}
          DefaultCacheBehavior:
            AllowedMethods:
              - GET
              - HEAD
            CachedMethods:
              - GET
              - HEAD
            Compress: true
            ForwardedValues:
              QueryString: false
              Cookies:
                Forward: none
            DefaultTTL: 3600
            MaxTTL: 86400
            MinTTL: 0
            # PathPattern: String
            # SmoothStreaming: Boolean
            TargetOriginId: s3-origin
            # TrustedSigners:
            #   - String
            ViewerProtocolPolicy: redirect-to-https
        Tags: ${self:custom.Tags}
    DnsRecords:
      Condition: isProdOrRC
      Type: "AWS::Route53::RecordSetGroup"
      Properties:
        HostedZoneName:
          Fn::If:
            - isRC
            - demo.com.
            - ${self:custom.domain}.
        Comment: "DNS records associated with ${self:custom.stackname}"
        RecordSets:
          - Name: ${self:custom.domain}
            Type: A
            AliasTarget:
              DNSName:  {"Fn::GetAtt": [CDN, DomainName]}
              HostedZoneId: Z2FDTNDATAQYW2

resources:
  # using conditions
  Conditions:
   isProd: {"Fn::Equals" : ["${self:custom.stage}", "prod"]}
   isDev: {"Fn::Equals" : ["${self:custom.stage}", "dev"]}
   isProdOrDev: {"Fn::Or": [{"Condition": "isProd"}, {"Condition": "isDev" }]}
  ExampleConditionalProp: {"Fn::If" : [ isProd, 'is-prod', 'not-prod']}
  ExampleConditionalPropNoValue: {"Fn::If" : [ isProd, 'is-prod', { Ref: AWS::NoValue }]}

More!

  #### Cloudformation Conditions ####
  Conditions:
    IsProd: {"Fn::Equals": ["${self:provider.stage}", "prod"]}
    IsStaging: {"Fn::Equals": ["${self:provider.stage}", "staging"]}
    IsDev: {"Fn::Equals": ["${self:provider.stage}", "dev"]}
    IsIntegration: {
      "Fn::And": [
          {"Fn::Not": [{"Condition": "IsProd"}] },
          {"Fn::Not": [{"Condition": "IsStaging"}] },
          {"Fn::Not": [{"Condition": "IsDev"}] },
        ]
      }
    IsProdOrStaging: {"Fn::Or": [{ "Condition": "IsProd"}, {"Condition": "IsStaging" }]}
    # Check for SES ARN for cognito email sending
    HasCognitoEmailSenderArn: {
        "Fn::Not": [{ "Fn::Equals" : ["", { Ref: CognitoEmailSenderArn }] }]
      }