Fix: no-restricted-imports schema allows multiple paths/patterns objects

[mdjermanovic]

What is the purpose of this pull request? (put an "X" next to item)

[X] Bug fix
[X] Other, please explain:

At the moment, no-restricted-imports schema allows multiple objects with paths/patterns keys:

/* eslint "no-restricted-imports": ["error", {
    "paths": ["foo"]
}, {
    "paths": ["bar"]
}] */

import a from "bar"; // no errors

Online Demo Link

This shouldn't be allowed because it isn't documented and it doesn't work as the user might expect. paths and patterns are being read only from the first object (at index 0), others are ignored.

What changes did you make? (Give an overview)

Fixed the schema to allow only 1 object with this form.

Also added a test to ensure that a configuration without any specified restricted imports is still allowed.

Is there anything you'd like reviewers to focus on?

This isn't related to #12638.

Marked as fix rather than chore because this change might break a build with an invalid configuration, which anyway didn't work as the user was expecting. Also, looks more like a fix than an update?

[kaicataldo]

LGTM, thanks. Agreed that this should be a semver-patch bug fix.

[btmills]

LGTM. Agreed this can be semver-minor.