Set `pdx_metadata_score` to zero for non PDX models

[mauroz77]

Description

To make sorting more straightforward, and to offer a consistent JSON structure for the scores of all models, all rows will have the same number of scores, regardless of the model characterization not applying to all models. If the model characterization does not apply to a model type, the corresponding score will be zero.

Before:

"scores": {
    "data_score": 42
}

After:

"scores": {
    "data_score": 42,
    "pdx_metadata_score": 0
}

[mauroz77]

@zperova done

[mauroz77]

@zperova closing to unblock dependent ticket