statemachine: Avoid warning when setting up signal transition

If the sender object was set, but not the signal signature, the
registration would proceed anyway, producing a strange warning like

QSignalTransition: no such signal: MyObject::

Change-Id: If0b113bdb60dd770d60b0d38d509b673e9d8c5eb
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>

src/corelib/statemachine/qstatemachine.cpp

@@ -1692,6 +1692,8 @@ void QStateMachinePrivate::registerSignalTransition(QSignalTransition *transitio
     if (!sender)
         return;
     QByteArray signal = QSignalTransitionPrivate::get(transition)->signal;
+    if (signal.isEmpty())
+        return;
     if (signal.startsWith('0'+QSIGNAL_CODE))
         signal.remove(0, 1);
     const QMetaObject *meta = sender->metaObject();