`AccessMember.evaluate` should always return `undefined` for non-existing members

[nenadvicentic]

The Issue

Current version of aurelia-bindinig@2.5.5 resolves value of non-existing member of an object differently when the object is null vs undefined or non existant.

For example, binding expression foo.bar would resolve to null for:

{ foo: null }

but would resolve to undefined for:

{ foo: undefined }

and

{}

while if foo is an empty object, binding expression foo.bar would correctly return undefined:

{ foo: {} }

This unpredictability produces issue in some binding scenarios. For example:

<input type="radio" model.bind="null" checked.bind="foo.bar" />
<input type="radio" model.bind="false" checked.bind="foo.bar" />
<input type="radio" model.bind="true" checked.bind="foo.bar" />

The culprit is this method:

binding/src/ast.js

Lines 251 to 254 in ed35588

	evaluate(scope, lookupFunctions) {
	let instance = this.object.evaluate(scope, lookupFunctions);
	return instance === null || instance === undefined ? instance : instance[this.name];
	}

Instead of returning instance, it should return undefined:

evaluate(scope, lookupFunctions) {
    let instance = this.object.evaluate(scope, lookupFunctions);
    return instance === null || instance === undefined ? undefined: instance[this.name];
  }

Expected behavior

foo.bar binding expression should return undefined for all 4 cases:

{ foo: null }
{ foo: undefined }
{ foo: {} }
{}

It would also be consistant with result of JavaScript optinal chaning syntax this.foo?.bar.