Support functions without events in CloudFront remove logging

[pmuens]

What did you implement:

Fixes a bug where a function with 0 events caused the removal to fail.

How did you implement it:

Add a check if the function has events defined in the CloudFront removal logger. Also added a regression test.

How can we verify it:

Deploy and remove the following service (the removal fails without this fix):

service: test-${self:custom.idx}

provider:
  name: aws
  runtime: nodejs10.x
  versionFunctions: false

custom:
  idx: 0

functions:
  hello:
    handler: handler.greeter

Todos:

Note: Run npm run test-ci to run all validation checks on proposed changes

• Write tests and confirm existing functionality is not broken.
  Validate via npm test
• Ensure there are no lint errors.
  Validate via npm run lint-updated
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• Ensure introduced changes match Prettier formatting.
  Validate via npm run prettier-check-updated
  Note: All reported issues can be automatically fixed by running npm run prettify-updated
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO