macosx: fix an exception when library input item is nil
requested to merge umxprime/vlc:umxprime/4.x/macosx/fix-thrown-assert-when-playback-continuation-is-called/mr into master
Given libraryMediaItem is nil in this context, an assertion was
thrown in the next resumePlaybackPositionForLibraryMediaItem:
call.
This patch make the code return when libraryMediaItem is nil to prevent the assertion.