By coincidence I asked my brother in law this question just a few weeks ago. He has worked his whole life on solar panels, designs them and improves them.
Asked him exactly the same question: 'during the winter our solar panels are not delivering any charge to the batteries since we are connected to shore power. Can this harm the panels ?
His answer was simple. If the panels do not deliver any charge (so no amps flowing) the only thing that will happen is that they will warm up a bit. However, since they will never reach temperatures of 60 to 70 degrees (Celsius) and only at those temperatures you would see degradation of the crystals, during winter they cannot be damaged.
Yes, there will be Voltage on the cables, but that is about it. He explained it to me as an electricity outlet in the wall, which is not in use, where you will also find a Voltage, but no amps.
If you would do this during the summer heat, peak of the day, you could reach that high temperature and thus degradation of the panels. So it is always good to have some airflow over the panels to cool them down.