Experimentally disable null expectations for throws assertions

[JSimoni42]

Aims to fix #2410

[novemberborn]

Thanks for picking this up @JSimoni42!

[novemberborn]

I think it'll be better if we keep this clause, but then before assigning expectations = {} we check experiments.disableNullExpectations && expectations === null. It's OK to duplicate the code to throw the assertion error.

The easier it is to understand the experimental logic, the easier to upgrade the codebase to make it the default logic.

[novemberborn]

Awesome.

What's left now is to update the documentation:

• https://github.com/avajs/ava/blob/master/docs/03-assertions.md#throwsfn-expectation-message
• https://github.com/avajs/ava/blob/master/docs/03-assertions.md#throwsasyncthrower-expectation-message

In both those places it says you need to specify null. Since we now allow undefined as well let's just recommend that. And then we can say something in parentheses like:

(AVA 3 also allows you to specify null. This will be removed in AVA 4. You can opt in to this change early by enabling the disableNullExpectations experiment.)

[JSimoni42]

@novemberborn Documentation has been updated. I noticed that #2575 was also failing checks due to the same test error in Install and test AVA / Node.js (^14.0.0, ubuntu-latest). Suffice it to say, it seems like this PR didn't introduce any additional test failures. Let me know if I should squash/merge or if you'd prefer to handle that, yourself

[novemberborn]

Let me know if I should squash/merge or if you'd prefer to handle that, yourself

Should be fine now…

[novemberborn]

Thanks @JSimoni42!