feat(events): make target optional in onXxx() methods

[jogold]

Make target optional in OnEventOptions so that it is no required when calling a onXxx()
method.

This allows to use "preconfigured" rules in other constructs.

Update awslint:events-method-signature to enforce optional options parameter in onXxx() method signatures.

Closes #2913

---

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)

---

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

[eladb]

Is it valid to provision a Rule without any targets? If not, then maybe we want to implement “validate” at the rule level?

Test for this?

[jogold]

Is it valid to provision a Rule without any targets?

Yes https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-events-rule.html#cfn-events-rule-targets

[eladb]

Okay, so let’s just add a test

[eladb]

It’s fishy that nothing broke at the consumption side when this changes from required to optional. I’d expect some null check...

[jogold]

It’s fishy that nothing broke at the consumption side when this changes from required to optional. I’d expect some null check...

https://github.com/awslabs/aws-cdk/blob/83eee09e8198ae1921b932c09a6d14c88392800d/packages/%40aws-cdk/aws-events/lib/rule.ts#L115-L127

There's one already

[jogold]

Okay, so let’s just add a test

https://github.com/awslabs/aws-cdk/blob/83eee09e8198ae1921b932c09a6d14c88392800d/packages/%40aws-cdk/aws-events/test/test.rule.ts is full of rules created without targets, what kind of test do you want to add? something in the onXxx() method of another package ?

An extra test in rds without target?
https://github.com/awslabs/aws-cdk/blob/83eee09e8198ae1921b932c09a6d14c88392800d/packages/%40aws-cdk/aws-rds/test/test.instance.ts#L315-L319

[eladb]

Yes sounds good. Just to exercise this specific path