Wrong get_control_deriv for LockedAmplitude

[goerz]

Via @mrs504aa in https://github.com/orgs/JuliaQuantumControl/discussions/64:

julia> using QuantumPropagators.Amplitudes: LockedAmplitude, ShapedAmplitude

julia> using QuantumControlBase: get_control_deriv

julia> shape(t) = 1.0
shape (generic function with 1 method)

julia> ampl1 = LockedAmplitude(shape)
LockedAmplitude(::typeof(shape))

julia> get_control_deriv(ampl1, shape)
ERROR: type LockedContinuousAmplitude has no field control

The correct result for any call to get_control_deriv with a LockedAmplitude is 0.0 (as the amplitude does not depend on any control, by definition)