Make User property conditionally nullable

Modified the User property in Envelope.cs to be conditionally nullable using the nullable preprocessor directive. This change allows the property to be nullable when the nullable directive is defined, enhancing flexibility and compatibility with different C# versions and project configurations.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -129,7 +129,11 @@ namespace EnvelopeGenerator.Domain.Entities
         public int? ExpiresWarningWhenDays { get; set; }
 
         [ForeignKey("UserId")]
-        public User User { get; set; }
+        public User
+#if nullable
+            ?
+#endif
+            User { get; set; }
 
         [Column("TFA_ENABLED")]
         public bool? TfaEnabled { get; set; }