awful: don't try to spawn nil or empty commands

lib/awful.lua.in

@@ -765,8 +765,10 @@ end
 -- @param cmd The command.
 -- @return The os.execute() return value.
 function spawn(cmd)
+    if cmd and cmd ~= "" then
         return os.execute(cmd .. "&")
     end
+end
 
 --- Eval Lua code.
 -- @return The return value of Lua code.