fix(core): Make filterUndefined null-safe

[RomainMuller]

There are a couple of places where fields accept values that are typed
as Json per the JSII type specification. This conveys that literal
null values may be passed and need to be preserved (as far as JSII is
concerned - see aws/jsii#523). However in the CloudFormation domain,
null is semantically equivalent to undefined.

Now enters Javascript's confusing type system, where null is an
object that cannot be converted to object (you read this correctly):

typeof null === 'object'
// => true

Object.entries(null);
// => Thrown:
//    TypeError: Cannot convert undefined or null to object
//        at Function.entries (<anonymous>)

So this changes the undefined checks to the null-coercing way, so
that null and undefined are handled the same way.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • 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
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which 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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