Skip good hostname check for undefined hostnames

Skip good hostname check for undefined hostnames

2016-02-17 13:00:50 #58424(normal) <a41bdc>; task; lua_metric_symbol_callback: call to (ONCE_RECEIVED) failed: /usr/local/share/rspamd/lua/once_received.lua:82: bad argument #1 to 'lower' (string expected, got nil); trace: [1]:{[C]:-1 - lower [C]}; [2]:{/usr/local/share/rspamd/lua/once_received.lua:82 - <unknown> [Lua]};