SessionManager: Clean up methods around projects

Clean up methods around projects() and update its usages to modern
language standards.

Change-Id: Ia0a4574d014ce41e0d511a392e4eed4680981d0e
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/vcsbase/vcsbaseeditor.cpp

@@ -1257,7 +1257,7 @@ static QTextCodec *findProjectCodec(const QString &dir)
                     return codec;
                 }
     }
-    return 0;
+    return nullptr;
 }
 
 QTextCodec *VcsBaseEditor::getCodec(const QString &source)