subsys: fs/nvs: do not change flash protection for zero-length case
subsys: fs/nvs: do not change flash protection for zero-length case

Avoid unsetting and setting the flash protection if there is nothing to
write to the flash. This happens for example when deleting data from the
flash using nvs_delete.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

explora26/zephyr,nashif/zephyr,kraj/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,finikorg/zephyr