Fixed behavior of Url with empty query

[kzrnm]

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• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
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Description

Fix #1015

Fixed behavior of Url.SearchParams with empty query

old behavior

var url = new AngleSharp.Dom.Url("http://example.com/foo/bar");
Console.WriteLine(url.SearchParams.ToString()); // "="

new behavior

var url = new AngleSharp.Dom.Url("http://example.com/foo/bar");
Console.WriteLine(url.SearchParams.ToString()); // ""

Assigning null to Url.Query

old behavior

var url = new AngleSharp.Dom.Url("http://example.com/foo/bar");
Console.WriteLine(url.ToString());      // "http://example.com/foo/bar"
Console.WriteLine(url.Query ?? "null"); // null
url.Query = null;
Console.WriteLine(url.ToString());      // "http://example.com/foo/bar?"
Console.WriteLine(url.Query ?? "null"); // ""

new behavior

var url = new AngleSharp.Dom.Url("http://example.com/foo/bar");
Console.WriteLine(url.ToString());      // "http://example.com/foo/bar"
Console.WriteLine(url.Query ?? "null"); // null
url.Query = null;
Console.WriteLine(url.ToString());      // "http://example.com/foo/bar"
Console.WriteLine(url.Query ?? "null"); // null

[CLAassistant]

All committers have signed the CLA.

[FlorianRappl]

LGTM - thanks! 🍻