Update ControlSignatureSniff.php

I got "Notice: Uninitialized string offset: 0 in /Users/username/.composer/vendor/pragmarx/laravelcs/Standards/Laravel/Sniffs/ControlStructures/ControlSignatureSniff.php on line 84"
Added isset on line 84
Merge pull request #4 from franzyke/master

Update ControlSignatureSniff.php