Update Context.php

Should return null instead of an empty string when context is not found. Handlebars.js returns undefined, which makes it possible to distict an empty string and not existing. (should not affect existing use, because null is handeled the same way as empty string)
Merge pull request #79 from ulriklystbaek/patch-5

Update Context.php, Return null instead of empty string when the variable is not available in context