power: test_power: correctly handle empty writes

Writing 0 length data into test_power makes it access an invalid array
location and kill the system.

Fixes: f17ef9b2

 ("power: Make test_power driver more dynamic.")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>