Empty response now triggers json exception (1.4.4)

[tractorcow]

We've discovered a critical breakage in our behat CI caused by the recent 1.4.4 release.

I've traced this to #67.

An example broken build can be seen at https://travis-ci.org/silverstripe/silverstripe-framework/jobs/248559035

Our selenium server log shows the requests being passed through to it without any apparent error.

10:32:36.017 INFO - Done: /session/96d209e0-a12b-43ae-aa50-eff91550202d/window/current/size

The issue is caused in AbstractWebDriver when $rawResult is empty. It looks as though selenium emits an empty body for this request. Previously in 1.4.3 this would incorrectly attempt to parse this as json, but fail silently.

In 1.4.4 this will incorrectly attempt to parse this as json, but raise an error.

What should probably happen is that empty results aren't parsed at all, but left empty.

We are using selenium 2.41.0.

[tractorcow]

Fix with tests at #79