fix: Check for null pointers when using R_ExternalPtrAddr directly

[paleolimbot]

Closes #281

In most of the package we use Rcpp, which throws for this case:

library(s2)

geog <- as_s2_geography("POINT (0 1)")
s2_area(unserialize(serialize(geog, NULL)))
#> Error: external pointer is not valid

^{Created on 2025-05-28 with reprex v2.1.1}

...but the print() goes through wk_format(), which used the C API directly.