QFileSystemWatcher: use nullptr, instead of 0

I'm not sure why this hasn't been flagged so far, probably
we don't have this warning enabled on MSVC.

Task-number: QTBUG-87713
Change-Id: I97c65079c8f8e439645ff7fe75eede9b01b26166
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/io/qfilesystemwatcher_win.cpp

@@ -596,7 +596,7 @@ QStringList QWindowsFileSystemWatcherEngine::removePaths(const QStringList &path
         }
     }
 
-    threads.removeAll(0);
+    threads.removeAll(nullptr);
     return unhandled;
 }