Tools - Make Entities and Repositories nullable (#3313)

* support nullability in tools' entities and repositories

* enables C# nullability checks in these files
* includes documentation for affected files

* refine documentation per code review

* improve comments on SendFileData structure

* fix ReferenceEvent.MaxAccessCount documentation

* add value notation to SendFileData.FileName

src/Core/Tools/Models/Data/SendData.cs

@@ -1,15 +1,33 @@
-namespace Bit.Core.Tools.Models.Data;
+#nullable enable
 
+namespace Bit.Core.Tools.Models.Data;
+
+/// <summary>
+/// Shared data for a send
+/// </summary>
 public abstract class SendData
 {
+    /// <summary>
+    /// Instantiates a <see cref="SendData"/>.
+    /// </summary>
     public SendData() { }
 
-    public SendData(string name, string notes)
+    /// <inheritdoc cref="SendData()" />
+    /// <param name="name">User-provided name of the send.</param>
+    /// <param name="notes">User-provided private notes of the send.</param>
+    public SendData(string name, string? notes)
     {
         Name = name;
         Notes = notes;
     }
 
-    public string Name { get; set; }
-    public string Notes { get; set; }
+    /// <summary>
+    /// User-provided name of the send.
+    /// </summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// User-provided private notes of the send.
+    /// </summary>
+    public string? Notes { get; set; } = null;
 }