Serializing and deserializing fastmap with no holes doesn't work right

[wch]

A new fastmap filled with exactly 32 items will have no holes. When serialized and unserialized, the resulting object provides the wrong result for as_list():

x <- seq_len(32)
m <- fastmap()
m$mset(.list = setNames(x, as.character(x)))
m1 <- unserialize(serialize(m, NULL))

m1$as_list()
#> named list()

This is because of this line:

fastmap/R/fastmap.R

Line 435 in 1d7890c

idxs <- seq_along(keys_)[-holes]

This line is supposed to remove the original vector with n_holes items removed. However, when n_holes is 0, then holes is a length-0 vector, and x[-integer(0)] will return an empty vector instead of what we want, which is just the original x unchanged.