ipc: rpmsg_service: Support IPM without data transfer

RPMsg currently sends four dummy bytes over the IPM when sending a
notification/interrupt to another CPU. However, most IPM drivers
support sending messages with no data transfer, and for some IPM
drivers, like the STM32 HSEM IPM driver, data transfer is not
possible at all, thus breaking compatibility. To fix this, send an
empty message with size 0 instead of a dummy message if the STM32
HSEM IPM driver is active.

Signed-off-by: Celina Sophie Kalus <hello@celinakalus.de>