Update error handling and initialization methods

- Add `using System;` to `CountryCodeHelper.cs`
- Throw `ArgumentException` for invalid country codes
- Initialize `Series` property with an empty array in `PlayoffSeriesSchedule.cs`

Nhl.Api.Common/Helpers/CountryCodeHelper.cs

@@ -1,4 +1,6 @@
-namespace Nhl.Api.Common.Helpers;
+using System;
+
+namespace Nhl.Api.Common.Helpers;
 
 /// <summary>
 /// A helper class for converting ISO standard country codes to the full country name
@@ -1247,6 +1249,6 @@ public static string ConvertThreeDigitCountryCodeToFullCountryName(string countr
             return "Zimbabwe";
         }
 
-        return string.Empty;
+        throw new ArgumentException($"Country code {countryCode} not found");
     }
 }

Nhl.Api.Domain/Models/Schedule/PlayoffSeriesSchedule.cs

@@ -117,7 +117,7 @@ public class Round
     /// Example: new List<Series> { /* Series objects */ }
     /// </summary>
     [JsonProperty("series")]
-    public List<Series> Series { get; set; } = new List<Series>();
+    public List<Series> Series { get; set; } = [];
 }
 
 /// <summary>

Nhl.Api.Tests/AssemblyInfo.cs

@@ -1,6 +1,6 @@
-[assembly: Parallelize(Workers = 4, Scope = ExecutionScope.MethodLevel)]
+[assembly: Parallelize(Workers = 4, Scope = ExecutionScope.MethodLevel)]
 namespace Nhl.Api.Tests;
 
-internal class AssemblyInfo
+internal sealed class AssemblyInfo
 {
 }