Ensure `resolve-link` handles undefined input

[johno]

TypeError: Cannot read property 'match' of undefined
    at resolveUrl (/var/www/cssstats/node_modules/get-css/utils/resolve-url.js:4:10)
    at createLink (/var/www/cssstats/node_modules/get-css/utils/create-link.js:6:10)
    at Object.<anonymous> (/var/www/cssstats/node_modules/get-css/index.js:53:25)
    at exports.each (/var/www/cssstats/node_modules/cheerio/lib/api/traversing.js:294:24)
    at parseHtml (/var/www/cssstats/node_modules/get-css/index.js:51:27)
    at Request._callback (/var/www/cssstats/node_modules/get-css/index.js:133:7)
    at Request.self.callback (/var/www/cssstats/node_modules/request/request.js:198:22)
    at emitTwo (events.js:87:13)
    at Request.emit (events.js:172:7)
    at Request.<anonymous> (/var/www/cssstats/node_modules/request/request.js:1063:14)