Update Client.php

[raphaelcangucu]

Make it compatible to HyGraph Graphql API

Description

I need to add a new parameter to the stream context to get the error content from the response body of the HyGraph api.
Also change to do not sent the variables parameter when its empty.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation