Allow trailing comma in box::use()

This commit allows empty expressions to be passed to `box::use()`, which
causes a trailing comma to be legal.

Resolves #172.

R/use.r

@@ -223,6 +223,8 @@ use = function (...) {
 #' @keywords internal
 #' @name importing
 use_one = function (declaration, alias, caller) {
+    # Permit empty expression resulting from trailing comma.
+    if (identical(declaration, quote(expr =)) && identical(alias, '')) return()
     spec = parse_spec(declaration, alias)
     info = rethrow_on_error(find_mod(spec, caller), sys.call(-1L))
     load_and_register(spec, info, caller)

tests/testthat/test-basic.r

@@ -109,3 +109,8 @@ test_that('partial name causes error', {
     expect_s3_class(error, 'simpleError')
     expect_identical(error$call, quote(a$doubl))
 })
+
+test_that('trailing comma is accepted', {
+    expect_error(box::use(mod/a, ), NA)
+    expect_error(box::use(mod/a, mod/b, ), NA)
+})