i2c_ll_stm32: Allow to send zero length message
i2c_ll_stm32: Allow to send zero length message

Some applications, like i2c_scanner, require to
send zero length messages.

Signed-off-by: Yannis Damigos <bda985a2727ac24c41652a294292988d6fc7dc73@gmail.com>

Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr