Simplify object notation for single-filter usage

[wol-soft]

Currently the library forces the user to define a list when using filter options even if only a single filter is applied:

{
    "type": "object",
    "properties": {
        "created": {
            "type": "string",
            "filter": [
                {
                    "filter": "dateTime",
                    "denyEmptyValue": true
                }
            ]
        }
    }
}

Instead of the list allow the direct usage of an object to define a single filter with options:

{
    "type": "object",
    "properties": {
        "created": {
            "type": "string",
            "filter": {
                "filter": "dateTime",
                "denyEmptyValue": true
            }
        }
    }
}

The above currently crashes with a message unsupported filter denyEmptyValues which is not helpful.