Replace null checks with IsEmpty property for ReadOnlySpan<char>

[prozolic]

This PR replace null checks with IsEmpty property for ReadOnlySpan<char>.
The change suppresses CA2265 warnings.

[Copilot]

Pull request overview

This PR aims to replace null checks with the IsEmpty property for ReadOnlySpan<char> variables to suppress CA2265 warnings. The changes update the LinkHelper.Urilize method to use span-based empty checks instead of null checks.

• Replaces null assignment with ReadOnlySpan<char>.Empty for the normalized variable
• Changes null check from normalized != null to !normalized.IsEmpty

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[MihaZupan]

Are we missing test coverage given that a null normalized would throw here?

[prozolic]

@MihaZupan Thank you for the review.
Since normalized is a ReadOnlySpan<char>, I changed the check from normalized != null to !normalized.IsEmpty.
When CharNormalizer.ConvertToAscii(c) returns null, it is implicitly converted to an empty span, maintaining the same behavior as before.
I confirmed from the following test cases.

https://github.com/xoofx/markdig/blob/master/src/Markdig.Tests/TestLinkHelper.cs#L330-L337

[MihaZupan]

Right, I was more referring to whether there are cases that could trigger an exception to be thrown before your current change. If so, it'd be good to have a test case for it.

[prozolic]

At this point, I cannot find any cases where an exception would be thrown before my change.
If an empty string ("") were assigned to normalized, the results of normalized != null and !normalized.IsEmpty would differ, which could cause an exception. However, I cannot find any cases where CharNormalizer.ConvertToAscii would return an empty string ("").
Does this response address your concern?