Update Culture model and DI for non-nullable properties

Refactored Culture model to use required and non-nullable properties.
Removed nullable checks for Info in ShowEnvelope.cshtml.
Changed DI from Cultures to MultiCulture in _ViewImports.cshtml.

EnvelopeGenerator.Web/Models/Culture.cs

@@ -4,15 +4,20 @@ namespace EnvelopeGenerator.Web.Models
 {
     public class Culture
     {
-        private string _language = string.Empty;
-        public string Language { get => _language;
-            init {
+        private string _language = null!;
+
+        public required string Language
+        {
+            get => _language;
+            init
+            {
                 _language = value;
                 Info = new(value);
             } 
         }
-        public string FIClass { get; init; } = string.Empty;
 
-        public CultureInfo? Info { get; init; }
+        public string FIClass { get; init; } = null!;
+
+        public CultureInfo Info { get; init; } = null!;
     }
 }