Invalidate returns with undefined values if no error is set

[caiges]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature

What is the current behavior? (You can also link to an open relevant tickets here)

Null returns are not allowed.

What is the new behavior (if this is a feature change)?

Null returns are allowed.

Where should the reviewer start?

then/then.js

This is how I tested it

Existing tests covered the change.

Does this PR introduce a breaking change?

If yes:

• [ ] Major version changed

  Please check if the PR fulfills these requirements**

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [x] Version changed by at least patch version

[Ellisande]

Can you add a test to specifically have it return null and insure that it works? I'm sure you tested it in practice but it'd be nice to have a unit test that covers the change you made

[Ellisande]

Looks solid, thanks for the contribution!