rtc: sysfs: Bail out earlier if no new groups provided

When there is no new groups provided, no need to reallocate memory,
copy the old ones and free them in order to do nothing. Do nothing
instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250702073224.2684097-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>