RemoteAcces: fix null media description crash

If for some reason the provider is stuck, the description in the getProviderContent can be null if the underlaying MedialibraryItem.description hasn't yet been initialized.

This can happen by triggering the smd auth dialog from the remote access, leaving it untouched on the phone, and then from the remote access querying any list of directories, their description is stuck as long as the smb dialog is still opened.

Fixes #3037 (closed)