Allow non-zero "fan off" PWM value (#15709)

Marlin/Configuration_adv.h

Marlin/src/inc/Conditionals_post.h

Marlin/src/module/planner.cpp

config/default/Configuration_adv.h

config/examples/3DFabXYZ/Migbot/Configuration_adv.h

config/examples/ADIMLab/Gantry v1/Configuration_adv.h

config/examples/ADIMLab/Gantry v2/Configuration_adv.h

config/examples/AlephObjects/TAZ4/Configuration_adv.h

config/examples/Alfawise/U20-bltouch/Configuration_adv.h

config/examples/Alfawise/U20/Configuration_adv.h

config/examples/AliExpress/UM2pExt/Configuration_adv.h

config/examples/Anet/A2/Configuration_adv.h

config/examples/Anet/A2plus/Configuration_adv.h

config/examples/Anet/A6/Configuration_adv.h

config/examples/Anet/A8/Configuration_adv.h

config/examples/Anet/A8plus/Configuration_adv.h

config/examples/Anet/E16/Configuration_adv.h

config/examples/AnyCubic/i3/Configuration_adv.h

config/examples/ArmEd/Configuration_adv.h

config/examples/BIBO/TouchX/cyclops/Configuration_adv.h