Get `LinkInline.Title` of `""` rather than `null` for `[link](/uri)`

[RamType0]

We get LinkInline.Title of "" with CommonMark Example 443.

[link](/uri)

As LinkInline.Title is typed as string? rather than string, it is weird.

markdig/src/Markdig/Renderers/Html/Inlines/LinkInlineRenderer.cs

Lines 72 to 77 in 3535701

	if (renderer.EnableHtmlForInline && !string.IsNullOrEmpty(link.Title))
	{
	renderer.WriteRaw(" title=\"");
	renderer.WriteEscape(link.Title);
	renderer.WriteRaw('"');
	}

HtmlRenderer render correct result for [link](/uri) with current parse behavior, but it will generate incorrect result for [link](/uri "") instead.

[Akarinnnnn]

This line normalized nulls into empty string. Not sure will comment this out cause problems.

markdig/src/Markdig/Helpers/LinkHelper.cs

Line 414 in 3535701

title ??= string.Empty;