Ensures that null values aren't used to create a CompositeStringStringKey

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Resolves: #19609

Description

I haven't been able to replicate the problem, which looks from the reported issue that it bubbles up from Commerce. But it does look to be similar to the fix applied for #16753 in #17024, and this is a further case.

The problem is that the constructor of CompositeStringStringKey accepts null values but throws if they are passed. So we ned to make sure we use string.Empty as we did in the other case.

Testing

Visual inspection will need to suffice here I believe.

[Copilot]

Pull Request Overview

This PR ensures that null culture or segment values are replaced with string.Empty when instantiating CompositeStringStringKey, preventing constructor exceptions.

• Default null culture and segment to empty strings in key creation.
• Prevents runtime errors by sanitizing inputs before passing to the key constructor.

Comments suppressed due to low confidence (2)

src/Umbraco.PublishedCache.NuCache/Property.cs:243

• [nitpick] The variable name k is ambiguous. Consider renaming it to compositeKey or cacheKey for clarity.

        var k = new CompositeStringStringKey(culture ?? string.Empty, segment ?? string.Empty); // Null values are not valid when creating a CompositeStringStringKey.

src/Umbraco.PublishedCache.NuCache/Property.cs:243

• This behavior change isn’t covered by existing tests. Adding a unit test that passes null values for culture and segment would ensure this guard remains effective.

        var k = new CompositeStringStringKey(culture ?? string.Empty, segment ?? string.Empty); // Null values are not valid when creating a CompositeStringStringKey.

[nikolajlauridsen]

LGTM 👍