Handle the case where variables is null

[XhmikosR]

H:\progs\dev\find-unused-sass-variables>node cli.js ..\..\
Looking for unused variables
i Finding unused variables in "H:\progs"...
H:\progs\dev\find-unused-sass-variables\index.js:35
    variables.forEach((variable) => {
              ^

TypeError: Cannot read property 'forEach' of null
    at Object.findUnusedVars [as find] (H:\progs\dev\find-unused-sass-variables\index.js:35:15)
    at args.forEach (H:\progs\dev\find-unused-sass-variables\cli.js:33:29)
    at Array.forEach (<anonymous>)
    at Object.<anonymous> (H:\progs\dev\find-unused-sass-variables\cli.js:28:6)
    at Module._compile (module.js:635:30)
    at Object.Module._extensions..js (module.js:646:10)
    at Module.load (module.js:554:32)
    at tryModuleLoad (module.js:497:12)
    at Function.Module._load (module.js:489:3)
    at Function.Module.runMain (module.js:676:10)