Fix TypeError due to missing mail config

Fixes this:
```
ERROR: Cannot read property 'transport' of undefined                         
Checking for updates failed, your blog will continue to function.
If you get this error repeatedly, please seek help from http://ghost.org/forum.                                                                           

TypeError: Cannot read property 'transport' of undefined
    at updateCheckData (/home/jakob/devel/Ghost/core/server/update-check.js:72:132)
    at updateCheckRequest (/home/jakob/devel/Ghost/core/server/update-check.js:97:12)
    at /home/jakob/devel/Ghost/core/server/update-check.js:180:24
    at NearFulfilledProxy.when (/home/jakob/devel/Ghost/node_modules/when/when.js:501:43)
    at Promise._message (/home/jakob/devel/Ghost/node_modules/when/when.js:426:25)
    at Array.deliver [as 0] (/home/jakob/devel/Ghost/node_modules/when/when.js:319:7)
    at runHandlers (/home/jakob/devel/Ghost/node_modules/when/when.js:385:12)
    at Array.14 (/home/jakob/devel/Ghost/node_modules/when/when.js:352:5)
    at runHandlers (/home/jakob/devel/Ghost/node_modules/when/when.js:385:12)
    at drainQueue (/home/jakob/devel/Ghost/node_modules/when/when.js:836:3) 
```