allow .its to yield undefined

[kuceb]

fix #1531 allow .its to yield undefined
fix #3241 undefined does not retry

yes, this is a breaking change. But I think it makes a lot of sense
maybe this is not a breaking change, since this makes test code fail less.

• add tests
• test case for getter throwing
  remove else clause that won't get hit
• allow cypress to retry undefined subject / test

[brian-mann]

Debate: should using not.exist on a path that doesn't exist throw if a property is called on an undefined object?

const obj = {
  foo: {}
}

// should this pass or fail?
cy.wrap(obj).its('foo.bar.baz').should('not.exist')

I believe this should pass because its is a path, and if you're asserting that the path shouldn't exist, then it shouldn't matter whether or not bar exists on the subject.