Fix error when authProvider check methods are not async

[fzaninotto]

Problem

When a developer forgets to return a Promise in authProvider.checkAuth() or authProvider.checkError(), they see a cryptic error message:

Cannot read properties of undefined (reading 'then')

Solution

Although the doc (and typescript) specifies that these methods must be async, react-admin can be more permissive and not throw any error in this case, by using async methods instead of promises.

How To Test

Update the authProvider in the simple example to use sync methods:

    checkError: ({ status }) => {
        if (status === 401 || status === 403) {
            throw new Error();
        }
    },
    checkAuth: () => {
        if (localStorage.getItem('not_authenticated')) {
            throw new Error();
        }
    },

The simple example should still work.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• [ ] The PR includes unit tests (if not possible, describe why)
• [ ] The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

No tests or stories because this is a corner case.