[Fix] ASN: fix _FAIL symbol for when main symbol is disabled

[Fix] ASN: fix _FAIL symbol for when main symbol is disabled

Fix error:
lua_dns_resolver_callback: call to dns callback failed: /usr/share/rspamd/plugins/asn.lua:74: attempt to concatenate field 'symbol' (a boolean value); trace: [1]:{/usr/share/rspamd/plugins/asn.lua:74 - <unknown> [Lua]};

Which happen when the main asn symbol is disabled in asn.conf using
symbol = false;

While here add nostat flag for ASN symbols (we cannot use them as
spam/ham signs).