When no contours are found, `contour` should not return a ContourLevel of 0 lines?

[aabacchus]

The hierarchy of types/structs you have makes it possible to have a ContourLevel at an isovalue but containing 0 lines, which doesn't make sense. Then, the lack of error means this simple function definition

Contour.jl/src/Contour.jl

Line 94 in 26994c4

contours(x, y, z, levels; VT=nothing) = ContourCollection([contour(x, y, z, l; VT=VT) for l in levels])

gives you a ContourCollection of 1 ContourLevel, but that contour has no lines. Can this interface be improved?