Handle null comparands

[jnyrup]

The changes in #2643 triggered another analyzer.
Probably because the signature of Compare(T? x, T?) says that either of x and y could be null.
This change way the simplest way (in terms of lines of code) to satisfy the analyzers.

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[github-actions]

Qodana for .NET

It seems all right 👌

No new problems were found according to the checks applied

💡 Qodana analysis was run in the pull request mode: only the changed files were checked
☁️ View the detailed Qodana report

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[coveralls]

Pull Request Test Coverage Report for Build 9080908463

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.561%

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Totals
Change from base Build 9079371722:	0.0%
Covered Lines:	12026
Relevant Lines:	12210

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💛 - Coveralls