Remove NilClass parameters from AR fixture methods

[amomchilov]

Motivation

Fixture methods have 3 overloads to describe the different ways they can be called. Their current shape incorrectly allows to nils to be passed, like:

users(nil)
users(:alice, nil)
users(:alice, nil, nil)

This PR fixes these annotations to not allow such nils, which has the pleasant side-effect of making them much simpler.

See also #1871, #1895

Implementation

Uses the same approach I used to describe Module#private (sorbet/sorbet#10239).

Tests

Updated.

Validated against Shopify Core. Type-checking fails, but for a great reason: there's a few of new errors like:

T.cast is useless because Foo is already a subtype of Foo