Fix null reference exception in CacheValues.For when building the Com…

…positeStringStringKey (#17024)

* Fix null ref exeption based on IPropertyValue.Culture documentation

* Clarify comment

src/Umbraco.PublishedCache.NuCache/Property.cs

@@ -371,6 +371,12 @@ private class CacheValues : CacheValue
 
         public CacheValue For(string? culture, string? segment)
         {
+            // As noted on IPropertyValue, null value means invariant
+            // But as we need an actual string value to build a CompositeStringStringKey
+            // We need to convert null to empty
+            culture ??= string.Empty;
+            segment ??= string.Empty;
+
             if (culture == string.Empty && segment == string.Empty)
             {
                 return this;