CloudFormation: Fn::Equals fails when parameter value is empty string passed via CreateStack API

[kibiz0r]

When a CloudFormation parameter with an empty string value ("") is passed via the CreateStack API parameters list and that parameter is referenced in an Fn::Equals condition, the stack fails with:

Template error: every Fn::Equals object requires a list of 2 string parameters.

The same template succeeds when:

• The parameter has a non-empty value
• The empty string comes from the template's Default: field (parameter omitted from API call)

Reproduction:

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  KmsKeyId:
    Description: Test param
    Type: String
Conditions:
  CreateKmsKey:
    Fn::Equals:
      - !Ref KmsKeyId
      - ''
Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Condition: CreateKmsKey
    Properties:
      BucketName: test-empty-param-bucket

# FAILS
aws cloudformation create-stack \
  --stack-name test-empty \
  --template-body file://template.yaml \
  --parameters 'ParameterKey=KmsKeyId,ParameterValue='

# SUCCEEDS (non-empty value)
aws cloudformation create-stack \
  --stack-name test-nonempty \
  --template-body file://template.yaml \
  --parameters 'ParameterKey=KmsKeyId,ParameterValue=some-value'

Both work correctly against real AWS CloudFormation.

Likely cause: Empty string parameter values are being coerced to None in Python during parameter resolution, which then fails the string type check in the Fn::Equals evaluator.

LocalStack version: 4.14.1 (Pro)