Handle undefined function package.exclude

[pjsier]

I'm using a service with the package rules:

package:
  exclude:
    - node_modules/@serverless-chrome/**

and then re-including that for only one of the functions to keep the package sizes down with:

functions:
  hello_world:
    package:
      include:
        - node_modules/@serverless-chrome/**

Currently, when I do that I get the error Cannot read property 'Symbol(Symbol.iterator)' of undefined because the prepare function only sets defaults for package if it does not exist at all, and doesn't check that it has both include and exclude.

My workaround for that has been to add an arbitrary path to exclude, but I added a default empty array here in case exclude is undefined. Let me know if there's anything you'd like me to update on this!

[schickling]

Thanks for the PR. Can you add a code comment in the line above to briefly describe what the code below does?