check that INS_HNTCn_REF is non-zero for ESC,RPM or FFT notches

[tridge]

it is quite common when users setup notch filters with ESC or FFT notches to forget to set INS_HNTCn_REF to a non-zero value. When set to zero it becomes a single fixed notch which is almost certainly not what they want.
We either need an arming check, or we need to make the is_zero(ref) check in AP_Vehicle::update_dynamic_notch() only apply to the throttle based notch
The ref is only used for:

• throttle based notch
• RPM notch (as scale factor)
• fallback to throttle notch for ESC notch (edge case)