Hi,

if you wish to add None then do this

Script Include:

var arr = [];

// to add None
arr.push({
	"value":"",
	"displayValue":"-- None --"
});

var DWN = new GlideRecord("u_customer");
DWN.addQuery("u_active", true);
DWN.addQuery("u_name", "Customer Specific Type");
DWN.addQuery('u_customer',' b70c0f830f37330013a831ef68767e33');
DWN.query();
if (DWN.next()) {
	PARMS = DWN.u_parms.split(',');
	gs.info("PARMS " + PARMS.length + "--" + PARMS);
	for (var i = 0; i < PARMS.length; i++) {
		var obj = {};
		var choice = new GlideRecord('sys_choice');
		choice.addQuery('name', 'incident');
		choice.addQuery('language', 'en');
		choice.addQuery('element', 'contact_type');
		choice.addQuery('inactive', false);
		choice.addQuery('value', PARMS[i]);
		choice.query();
		if (choice.next()) {
			obj["value"] = choice.value.toString();
			obj["displayValue"] = choice.label.toString();
			arr.push(obj);
		}
	}
}

var result = {};
result["choices"] = arr;

return JSON.stringify(result);
},

Workspace client script:

function onClick(g_form) {

	var fields = [];
	fields.push({
		type: 'string',
		label: getMessage('Invoicer'),
		value: g_form.getDisplayValue('consumer'),
		readonly: true,
	});

	var ga = new GlideAjax('CDAInvoiceProcessor');
	ga.addParam('sysparm_name', 'getInvoiceTypes');
	ga.getXML(populateChoiceeField);
	ga.getXMLAnswer(populateChoiceeField);

	function populateChoiceeField(response) {
		var msg = response;
		var answer = JSON.parse(msg);
		fields.push({
			type: 'choice',
			name: 'invoice_type',
			label: getMessage('invoice type'),
			choices: answer.choices,
			mandatory: true
		});

		g_modal.showFields({
			title: "Product for Invoice",
			fields: fields,
			size: 'lg'
		}).then(function(fieldValues) {
			g_form.setValue('short_description', fieldValues.updatedFields[1].value);
			//<--------------------------------------------------------------------------------------->//	


			//<---------------------------------------------------------------------------------------->//	

		});
	}
}

Regards
Ankur