feat(spawn, lib: awful: spawn: with_line_callback): make callbacks optional

lib/awful/spawn.lua

@@ -123,8 +123,10 @@ function spawn.with_line_callback(cmd, callbacks)
             done_before = true
             return
         end
+        if done_callback then
             done_callback()
         end
+    end
     if have_stdout then
         spawn.read_lines(Gio.UnixInputStream.new(stdout, true),
                 stdout_callback, step_done, true)

spawn.c

@@ -402,7 +402,7 @@ luaA_spawn(lua_State *L)
         return_stdout = luaA_checkboolean(L, 4);
     if(lua_gettop(L) >= 5)
         return_stderr = luaA_checkboolean(L, 5);
-    if(lua_gettop(L) >= 6)
+    if (!lua_isnoneornil(L, 6))
     {
         luaA_checkfunction(L, 6);
         flags |= G_SPAWN_DO_NOT_REAP_CHILD;