Fix tune_args() error with namespaced function arguments (#261)

[EmilHvitfeldt]

Fixes #261.

Problem

Setting a model argument to a namespaced function, e.g. hier_clust(num_clusters = tune(), dist_fun = philentropy::distance), caused tune_cluster() / tune_args() to error with argument is of length zero.

The argument expression pkg::fun is a :: call, for which rlang::call_name() returns NULL. The check rlang::call_name(x) == "tune" then became NULL == "tune", which is length-zero and errors inside if().

Fix

In tune_id(), capture the call name once and guard against NULL:

nm <- rlang::call_name(x)
if (!is.null(nm) && nm == "tune") {

Added a test confirming tune_args() works with dist_fun = stats::dist, and a NEWS bullet.