Widen callable type to allow empty string default value

[IanDelMar]

Widens the callable-typed parameters to also allow the default value ''. Related: #289.

Affected:

• Custom_Background::__construct()
• Custom_Image_Header::__construct()

Also makes the callable type explicit by adding parameter and return types. These callables are either hooked to admin_head-{$page}, which does not pass any arguments, or passed to call_user_func() without arguments. As an action hook, admin_head-{$page} expects a callable with a void return type. call_user_func() is used in a template context where side effects are expected, but no return value is used. Hence, callable(): void.