Common class fallback seems to fail when "filling in" missing columns

[DavisVaughan]

library(vctrs)
library(tibble)

x <- tibble()
y <- tibble(a = structure(1L, class = "foo"))

vec_rbind(x, y)
#> Error: Can't combine `..1` <vctrs:::common_class_fallback> and `..2` <foo>.

^{Created on 2022-01-10 by the reprex package (v2.0.1)}