Stop updating name properties if all are already set

[brandonkelly]

Description

• NameTrait::prepareNamesForSave() no longer makes any changes if fullName and firstName/lastName are already set on the model.
• User::setAttributes(), Address::setAttributes(), and UsersController::populateNameAttributes() now unset firstName and lastName if fullName was posted (in addition to already unsetting fullName if firstName/lastName were posted).

With those changes, it’s now possible to load up a user/address and resave them programmatically without affecting the name properties.

Related issues

• [4.x]: Re-saving an address can mutate the first and last name on the address #14665