Don't warn about boxplot orientation when position aesthetics are missing

[teunbrand]

This PR amends #6631.

I spotted this in the following test in the CI, where the warning from #6631 popped up:

ggplot2/tests/testthat/test-stat-boxplot.R

Lines 23 to 27 in d896cad

	test_that("stat_boxplot errors with missing x/y aesthetics", {
	p <- ggplot(PlantGrowth) +
	geom_boxplot()
	expect_snapshot_error(ggplot_build(p))
	})

Arguably, when you're missing the x and y aesthetic, getting the orientation right is a secondary problem and distracts from the missing aesthetic problem.

[thomasp85]

LGTM