Fix 3+ nested Jet constructor

In Jet types, there is an attempt to set the derivative to 0.
However, if the derivative is not directly a scalar but a Jet<Jet>,
this cannot be constructed directly from a 0 literal. This is solved
by using the default constructor for the scalar type instead of 0.

The typical use case for this is adding constraints to the second
derivative of a curve in an Autodiff cost function.

Change-Id: Id480096632a731f312be12e294b3d6e244211529