safeguard against missing `arg`
safeguard against missing `arg`

When using LuaUnit from within other projects (i.e. not in
"standalone" mode), the variable `arg` may possibly be undefined,
causing an error for the `cmdline_argv` assignment.

This commit prevents that by using rawget(), which returns `nil`
if the variable is absent.

Signed-off-by: NiteHawk <5758ffa791ed0666fb75085c5624d5e6d66bbc05@users.noreply.github.com>

GuntherStruyf/luaunit,GuntherStruyf/luaunit