r.univar: Fix JSON structure

[NishantBansal2003]

Fixes: #5652

• JSON output without zones option:

{
            "n": 16200,
            "null_cells": 0,
            "cells": 16200,
            "min": 102,
            "max": 380,
            "range": 278,
            "mean": 241,
            "mean_of_abs": 241,
            "stddev": 62.04702517714555,
            "variance": 3849.8333333333335,
            "coeff_var": 25.745653600475332,
            "sum": 3904200,
            "first_quartile": 191,
            "median": 241,
            "third_quartile": 291,
            "percentiles": [{"percentile": 90, "value": 324}],
}

• JSON output with zones option:

[
            {
                "zone": 1,
                "zone_label": "",
                "n": 3420,
                "null_cells": 0,
                "cells": 3420,
                "min": 102,
                "max": 309,
                "range": 207,
                "mean": 205.5,
                "mean_of_abs": 205.5,
                "stddev": 56.611983419296187,
                "variance": 3204.9166666666665,
                "coeff_var": 27.548410423015174,
                "sum": 702810,
                "first_quartile": 155,
                "median": 205.5,
                "percentiles": [{"percentile": 90, "value": 282}],
                "third_quartile": 255,
            },
            {
                "zone": 2,
                "zone_label": "",
                "n": 12780,
                "null_cells": 0,
                "cells": 12780,
                "min": 121,
                "max": 380,
                "range": 259,
                "mean": 250.5,
                "mean_of_abs": 250.5,
                "stddev": 59.957623924457401,
                "variance": 3594.9166666666665,
                "coeff_var": 23.935179211360243,
                "sum": 3201390,
                "first_quartile": 200,
                "median": 250.5,
                "percentiles": [{"percentile": 90, "value": 330}],
                "third_quartile": 300,
            },
]

[petrasovaa]

Thanks, that fixes the issue. Could you please add a simple example in Python with parse_command? Maybe not at the end, but sooner, e.g. in the first example (univariate statistics).