fix(ssm): Generate correct SSM Parameter ARN

[RomainMuller]

The SSM parameter name (resourceName portion of it's ARN) is required
to start with a /, which caused the presence of a doubled // in the generated
ARN caused by the fact sep defaults to /.

This change allows sep to be set to an empty string, allowing SSM rendering
of correct SSM parameter ARNs.

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Pull Request Checklist

• Testing
  • Unit test added
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[eladb]

• missing unit test in @aws-cdk/cdk
• how does this behave for parse?

[RomainMuller]

So... the case for parse is a little awkward, because technically the separator here is /, but the SSM Parameter users would have to know that they have to re-add the / prefix to the resourceName part.

And I've added a test case for format.