Return a line that is tangent to the curve at point specified by
The function expects
t to lie between 0 and 1; values outside the range are constrained to 0 and 1, respectively.
The tangent line starts at the specified point. The direction from
end is the same as the direction of the curve at the specified point.
If the control points of the curve all lie at the same coordinates,
null is returned (it is impossible to determine the slope of a point). The
curve.isDifferentiable() function may be used in advance to determine whether tangents can exist for a given curve.