Missing contour parameter defaults to TRUE

[teunbrand]

This PR aims to fix #5571.

It is a simple fix that just does what the title indicates.

Example from linked issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot() +
  layer(
    geom = GeomDensity2d,
    stat = StatDensity2d,
    data = faithful,
    mapping = aes(x = eruptions, y = waiting),
    position = PositionIdentity,
  )

^{Created on 2024-12-09 with reprex v2.1.1}

[thomasp85]

LGTM