subsys/random: Hide 'Random generator' choice when empty
subsys/random: Hide 'Random generator' choice when empty

Prevent the choice from showing up in the menu when none of the choice
symbols are visible.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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