fcrdns: handle a 'null' hostname

Prevent this error, when a hostname of 'null' is encountered. Experienced in the wild. :(

[CRIT] [-] [core] TypeError: Cannot call method 'split' of null
[CRIT] [-] [core]     at Plugin.exports.is_generic_rdns (/usr/local/lib/node_modules/Haraka/plugins/connect.fcrdns.js:246:54)
[CRIT] [-] [core]     at Plugin.exports.check_fcrdns (/usr/local/lib/node_modules/Haraka/plugins/connect.fcrdns.js:179:20)
[CRIT] [-] [core]     at /usr/local/lib/node_modules/Haraka/plugins/connect.fcrdns.js:86:31
[CRIT] [-] [core]     at asyncCallback (dns.js:68:16)
[CRIT] [-] [core]     at Object.onanswer [as oncomplete] (dns.js:147:9)