Make it possible to create a session using an empty object

At minimum, the New Session command mandates the presence of a capabilities field with an empty object for creating a new session:

{"capabilities": {}}

This is due to how the processing capabilities algorithm is phrased:

1. Let capabilities request be the result of getting the property "capabilities" from parameters.
   1. If capabilities request is not a JSON object, return error with error code invalid argument.

The logical consequence of this is that using an empty top-level JSON Object, like this, is illegal:

{}

It was @whimboo who discovered this, and first reported it in https://bugzilla.mozilla.org/show_bug.cgi?id=1470976.

Start with the New Session command and its processing capabilities algorithm in the WebDriver specification, then review the linked Bugzilla report for the intended behavior. The work is done when an empty top-level JSON object is accepted for session creation without violating the capabilities-processing rules.