Wrong current Track fix

[daschuer]

This fixes an issue that the wrong track is listed in the History.

This fixes a regression since #3500
https://bugs.launchpad.net/mixxx/+bug/1933991

[LeDucDuBleuet]

I can confirm this fix is working well, the history log is now accurate. Thank you very much for the quick response!

[daschuer]

Ready.

[uklotzde]

I expect that no other component is waiting for a trackLoaded signal with a nullptr since we also have trackUnloaded. There is even code that handles this special case, e.g. AutoDJCratesDAO::slotPlayerInfoTrackLoaded(). Code like this should probably be cleaned up by a subsequent PR, just to make sure we didn't miss a use case. I'll take care of that.

Thank you! LGTM

[uklotzde]

@daschuer Maybe this merge was too fast. There are components that only listen to trackLoaded but not the trackUnloaded signal, e.g. BpmControl which expects to receive a trackLoaded(nullptr) signal to reset itself. Not sure if this actually matters.

Those redundant signals cause a mess and need to be cleaned up eventually.