Bugfix: crash when checking the sync file status

Sometimes a `Folder` is created for a local path that we cannot sync
(e.g. it's not writable). This folder will not have a sync engine. If
the engine of this `Folder` is accessed, this will lead to a crash.

The fix is to check for the existence of a sync engine in the
`Folder::canSync()` method.

https://github.com/owncloud/client/issues/11981
https://github.com/owncloud/client/pull/11982
https://github.com/owncloud/client/pull/12000
