Fix Nullable reference types

Author: tinohager

src/Nager.Date/HolidaySpecificationProcessor.cs

@@ -28,7 +28,7 @@ internal static IEnumerable<Holiday> Process(
                     EnglishName = holidaySpecification.EnglishName,
                     LocalName = holidaySpecification.LocalName,
                     HolidayTypes = holidaySpecification.HolidayTypes,
-                    SubdivisionCodes = holidaySpecification.SubdivisionCodes ?? [],
+                    SubdivisionCodes = holidaySpecification.SubdivisionCodes,
                     ObservedDate = holidaySpecification.ObservedRuleSet?.GetObservedDate(holidayDate) ?? holidayDate,
                     CountryCode = countryCode
                 };

src/Nager.Date/Models/Holiday.cs

@@ -42,7 +42,7 @@ public class Holiday
         /// The initial subdivision of the country (ISO 3166-2) where the holiday is observed
         /// </summary>
         /// <remarks>States, Province, Territories, Federal districts, Cantons</remarks>
-        public string[] SubdivisionCodes { get; set; } = [];
+        public string[]? SubdivisionCodes { get; set; }
 
         /// <summary>
         /// The types of the holiday