Accessing `-1` of a helper's positional arguments blows up

[boris-petrov]

🐞 Describe the Bug

Check the reproduction below.

🔬 Minimal Reproduction

import { helper } from '@ember/component/helper';

function returnUndefinedHelper(positionalArgs) {
  return positionalArgs[-1];
}

export default helper(returnUndefinedHelper);

😕 Actual Behavior

Calling that helper blows up instead of simply return undefined:

Uncaught (in promise) TypeError: Cannot destructure property 'tag' of 'ref' as it is undefined.
    at valueForRef (ember.js:1380:1)
    at PositionalArgsProxy.get (ember.js:1405:1)
    at returnUndefinedHelper (return-undefined.js:10:1)

🤔 Expected Behavior

No error.

🌍 Environment

• Ember: 6.4.0
• Ember-CLI: 6.4.9
• Node.js/npm:v23.11.1
• OS: Linux
• Browser: Chrome 137

[NullVoxPopuli]

looks like this is because we get valueForRef for any value of index passed to positional as long as the value is less than the length of the array.

ie: we forgot to check for >= 0 -- (in which case we can early return undefined as [-1] isn't supported) -- and valueForRef doesn't take undefined.

A repro is available here (and if you pause on exceptions, you can see what exactly went wrong in the glimmer-renderer code)