Replace `zeroGrob()` with `grid::nullGrob()`

[teunbrand]

To my understanding, ggplot2 use to have a nullGrob() function itself. At some point, {grid} also introduced a homonymous function, forcing ggplot2 to introduce zeroGrob() to avoid a namespace conflict (7f12f21f89ba51793bea9223c19e2a4fb02bd7b8). However, I think grid::nullGrob() does the same thing as ggplot2::zeroGrob(), namely: draw nothing. In principle, we could just use grid::nullGrob() instead.