Test prefix: plotters empty uses not present

This issue tracks inconsistency with the json libraries management in the plotter and other editors such as the library editor.

Description of the issue:

In the test prefix, the plotters contain a "uses" key targeting the libraries that are used.

When used we have:

    "uses": {
        "baselib": "plot/baselib/1"
    }

When no libraries are defined I was expecting to have something like this:

    "uses": {}

However we get no "uses" key in the dictionary at all. This is counter-intuitive and works at the opposite of what is done for the libraries editor where no libraries used mean having:

    "uses": {}

in the json file.