AngleAxis(UnitQuaternion) produces NaN derivatives

Similar to #96, constructing an AngleAxis from a UnitQuaternion produces NaN derivative elements when using ForwardDiff:

julia> using Rotations

julia> using ForwardDiff: Dual

julia> q = UnitQuaternion(Dual(1.0, 0.0), Dual(0.0, 0.0), Dual(0.0, 0.0), Dual(0.0, 0.0))
3×3 UnitQuaternion{Dual{Nothing,Float64,1}} with indices SOneTo(3)×SOneTo(3)(Dual{Nothing}(1.0,0.0), Dual{Nothing}(0.0,0.0), Dual{Nothing}(0.0,0.0), Dual{Nothing}(0.0,0.0)):
 Dual{Nothing}(1.0,0.0)  Dual{Nothing}(0.0,0.0)  Dual{Nothing}(0.0,0.0)
 Dual{Nothing}(0.0,0.0)  Dual{Nothing}(1.0,0.0)  Dual{Nothing}(0.0,0.0)
 Dual{Nothing}(0.0,0.0)  Dual{Nothing}(0.0,0.0)  Dual{Nothing}(1.0,0.0)

julia> AngleAxis(q)
3×3 AngleAxis{Dual{Nothing,Float64,1}} with indices SOneTo(3)×SOneTo(3)(Dual{Nothing}(0.0,NaN), Dual{Nothing}(1.0,NaN), Dual{Nothing}(0.0,NaN), Dual{Nothing}(0.0,NaN)):
 Dual{Nothing}(1.0,NaN)  Dual{Nothing}(0.0,NaN)  Dual{Nothing}(0.0,NaN)
 Dual{Nothing}(0.0,NaN)  Dual{Nothing}(1.0,NaN)  Dual{Nothing}(0.0,NaN)
 Dual{Nothing}(0.0,NaN)  Dual{Nothing}(0.0,NaN)  Dual{Nothing}(1.0,NaN)

Start by reproducing the AngleAxis(q) example with ForwardDiff and inspect the AngleAxis(UnitQuaternion) conversion entry point. Trace the zero-angle handling and verify that the conversion preserves finite derivative elements instead of producing NaN values.