Fails on empty CSS files

[anton-rudeshko]

stack:  Error: Undefined tree at ./empty.css: "" => undefined
    at Object.Comb.processString (/Users/rudeshko/.nvm/v0.8.25/lib/node_modules/csscomb/lib/csscomb.js:103:19)
    at Comb.processFile (/Users/rudeshko/.nvm/v0.8.25/lib/node_modules/csscomb/lib/csscomb.js:119:46)
    at Array.Promise._callCallbacks [as 189] (/Users/rudeshko/.nvm/v0.8.25/lib/node_modules/csscomb/node_modules/vow/lib/vow.js:200:56)
    at callFns (/Users/rudeshko/.nvm/v0.8.25/lib/node_modules/csscomb/node_modules/vow/lib/vow.js:458:35)
    at process.startup.processNextTick.process._tickCallback (node.js:245:9)

Empty files should be ignored or reported as warnings.

[mishanga]

WIP