DESCRIPTORS: Fix summing loop not iterating over the whole buffer

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Fix divide by zero occuring for null power input

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Add tests for null input

Signed-off-by: Gama <misept.dieseneuf@gmail.com>