Respect default setting for "syncsite" ("Empty cache" on resource save)

### What does it do ?

Takes care of the `syncsite_default` setting when updating a resource.

### Why is it needed ?

`syncsite` is never stored, unless you define a Form Customization rule doing so.

Before this PR, the behavior was that the "Empty cache" checkbox was checked if no Form Customization rule dealing with "syncsite" was found, not matter what was defined as `syncsite_default` setting.

This will cause issues since `getOne('Context')` does not `modContext::prepare()`... will update making use of `modX::getContext()`