Make boolean properties nullable in Envelope class

Changed `SendReminderEmails`, `UseAccessCode`, and `TfaEnabled`
properties from non-nullable `bool` to nullable `bool?` to allow
representation of `null` values. Added conditional initialization
of `TfaEnabled` to `false` for `NETFRAMEWORK` target.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -94,7 +94,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public string Language { get; set; }
 
         [Column("SEND_REMINDER_EMAILS")]
-        public bool SendReminderEmails { get; set; }
+        public bool? SendReminderEmails { get; set; }
 
         [Column("FIRST_REMINDER_DAYS")]
         public int? FirstReminderDays { get; set; }
@@ -114,7 +114,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public int? CertificationType { get; set; }
 
         [Column("USE_ACCESS_CODE")]
-        public bool UseAccessCode { get; set; }
+        public bool? UseAccessCode { get; set; }
 
         [Column("FINAL_EMAIL_TO_CREATOR")]
         public int? FinalEmailToCreator { get; set; }
@@ -132,7 +132,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public User User { get; set; }
 
         [Column("TFA_ENABLED")]
-        public bool TfaEnabled { get; set; }
+        public bool? TfaEnabled { get; set; }
 #if NETFRAMEWORK
             = false;
 #endif