Fix coord_radial() full circle bug

[teunbrand]

This PR aims to fix #5750.

Briefly, the piece that checks whether the (start, end) interval contained horizontal or vertical thetas sometimes normalised a full circle to have 0 interval. Full circles are special-cased now to always include any theta.

Reprex from issue:

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

penguins |>
  ggplot() +
  geom_point(
    aes(x = flipper_length_mm, 
        y = body_mass_g,
        col = species)) +
  theme_classic() +
  coord_radial(start = 90)
#> Warning: Removed 2 rows containing missing values or values outside the scale range
#> (`geom_point()`).

^{Created on 2024-03-06 with reprex v2.1.0}

[thomasp85]

LGTM