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Operator '??' cannot be applied to operands of type 'Guid' and 'string'

See original GitHub issue

When generating C# against an API that takes an array of guid on the query string, autorest outputs code that fails to compile

foreach (var _item in ids)
{
    _queryParameters.Add(string.Format("ids={0}", System.Uri.EscapeDataString(_item ?? string.Empty)));
}

_item ?? string.Empty causes

CS0019 Operator ‘??’ cannot be applied to operands of type ‘Guid’ and ‘string’

Seems to be the same root issue as #1648

The below swagger causes a repro on 1.0.0-Nightly20170212

{
    "swagger": "2.0",
    "info": {
        "version": "1.0",
        "title": "title"
    },
    "host": "localhost",
    "schemes": ["http"],
    "paths": {
        "/test": {
            "delete": {
                "tags": ["test"],
                "operationId": "deleteTest",
                "consumes": [],
                "produces": [],
                "parameters": [{
                    "name": "ids",
                    "in": "query",
                    "required": true,
                    "type": "array",
                    "items": {
                        "type": "string",
                        "format": "uuid",
                        "x-nullable": false
                    },
                    "collectionFormat": "multi",
                    "x-nullable": true
                }],
                "responses": {
                    "204": {
                        "description": "No Content"
                    }
                }
            }
        },
    },
    "definitions": {

    }
}