REST: Avoid making 'wpaccuracy' required in API requests

Some configs can have this particular argument absent in the request.
For example defining different accuracy levels:

$wgFlaggedRevsTags = [
'accuracy' => [ 'levels' => 1, 'quality' => 20, 'pristine' => 21 ],
];

In such a case, requiring this as a boolean fails hence it should not be
hard required. It's in this case optional.

Bug: T294544
Change-Id: I01c724fea680bb61599e20e8b5c1d6c1273dd90b
(cherry picked from commit 3667758c900589383d19c34a3d8f8d9f38264026)