`encode_unit()` should only check the `direction` argument if it's supplied

[hfrick]

This should show the message from the .default() method.

library(dials)
#> Loading required package: scales

encode_unit("foo")
#> Error in `encode_unit()`:
#> ! argument "direction" is missing, with no default

^{Created on 2026-02-23 with reprex v2.1.1}