Working directory gets reset when the path doesn't exist

[PhysSong]

I use a directory in my NTFS partition as LMMS working directory. Once I forgot to mount the partition and launch LMMS, the working directory resets without showing "Working directory" dialog.

This is a regression from #4211. canonicalPath function returns an empty string for non-existent paths and causes problem.
I think we can work around by checking for empty string, or use a function that doesn't return an empty string(absolutePath or QDir::cleanPath) in such situations.

In Qt5, QDir::cleanPath has an additional functionalitiy. It always use / for separators.

Edit: CleanPath to cleanPath

[tresf]

QDir::cleanPath() looks like a sane replacement.