cover trailing -- and os:argv([]) in os_argv test

tests/scryer/cli/issues/os_argv.stdin

@@ -2,3 +2,4 @@ os:argv(V).
 os:argv(["test1"|_]).
 os:argv(["--"|_]).
 os:argv(["test2"|_]).
+os:argv([]).

tests/scryer/cli/issues/os_argv.stdout

@@ -1,4 +1,5 @@
-   V = ["test1","--","test2"].
+   V = ["test1","--","test2","--"].
    true.
    false.
    false.
+   false.

tests/scryer/cli/issues/os_argv.toml

@@ -1,2 +1,2 @@
 # https://github.com/mthom/scryer-prolog/pull/2263#issuecomment-1874400820
-args = ["-f", "--no-add-history", "--", "test1", "--", "test2"]
+args = ["-f", "--no-add-history", "--", "test1", "--", "test2", "--"]