Fix crash when no features are available to calculate a layer transform

[luca-della-vedova]

When a there is a layer with a robot map but no features have been matched between the layer and the floor plan, an empty optimization problem is created and the ceres solver throws an exception crashing rmf_traffic_editor with the following backtrace:

F0202 15:27:15.213660 2532974 problem_impl.cc:552] Parameter block not found: 0x7ffec6fdc2b8. You must add the parameter block to the problem before you can set a lower bound on one of its components.
*** Check failure stack trace: ***
    @     0x7fb952a0cb03  google::LogMessage::Fail()
    @     0x7fb952a149d1  google::LogMessage::SendToLog()
    @     0x7fb952a0c7c2  google::LogMessage::Flush()
    @     0x7fb952a0e78f  google::LogMessageFatal::~LogMessageFatal()
    @     0x7fb952744fa9  ceres::internal::ProblemImpl::SetParameterLowerBound()
    @     0x55e3367651cc  Level::optimize_layer_transforms()
    @     0x55e336712588  Editor::edit_optimize_layer_transforms()
    @     0x7fb9510f1793  (unknown)
    @     0x7fb951f65be6  QAction::triggered()
    @     0x7fb951f688fc  QAction::activate()
    @     0x7fb9520f3142  (unknown)
    @     0x7fb9520fad39  (unknown)
    @     0x7fb951faf4ee  QWidget::event()
    @     0x7fb951f6c713  QApplicationPrivate::notify_helper()
    @     0x7fb951f74364  QApplication::notify()
    @     0x7fb9510b9e3a  QCoreApplication::notifyInternal2()
    @     0x7fb951f72e47  QApplicationPrivate::sendMouseEvent()
    @     0x7fb951fc9843  (unknown)
    @     0x7fb951fcbfd5  (unknown)
    @     0x7fb951f6c713  QApplicationPrivate::notify_helper()
    @     0x7fb9510b9e3a  QCoreApplication::notifyInternal2()
    @     0x7fb951741307  QGuiApplicationPrivate::processMouseEvent()
    @     0x7fb951716a2c  QWindowSystemInterface::sendWindowSystemEvents()
    @     0x7fb94bf01d6e  (unknown)
    @     0x7fb94f320d3b  g_main_context_dispatch
    @     0x7fb94f3756c8  (unknown)
    @     0x7fb94f31e3e3  g_main_context_iteration
    @     0x7fb9511130b8  QEventDispatcherGlib::processEvents()
    @     0x7fb9510b875b  QEventLoop::exec()
    @     0x7fb9510c0cf4  QCoreApplication::exec()
    @     0x55e33670c722  main
    @     0x7fb950229d90  (unknown)
Aborted

This PR just avoids calling ceres to do the optimization if no features are present, avoiding the crash