Enable nullable for more files and add tests

Adds basic tests for FFProbeVideoInfo.CreateDummyChapters
Fixed error message CreateDummyChapters instead of reporting the total minutes it only reported the minute component

MediaBrowser.Model/Entities/ChapterInfo.cs

@@ -1,4 +1,3 @@
-#nullable disable
 #pragma warning disable CS1591
 
 using System;
@@ -20,16 +19,16 @@ namespace MediaBrowser.Model.Entities
         /// Gets or sets the name.
         /// </summary>
         /// <value>The name.</value>
-        public string Name { get; set; }
+        public string? Name { get; set; }
 
         /// <summary>
         /// Gets or sets the image path.
         /// </summary>
         /// <value>The image path.</value>
-        public string ImagePath { get; set; }
+        public string? ImagePath { get; set; }
 
         public DateTime ImageDateModified { get; set; }
 
-        public string ImageTag { get; set; }
+        public string? ImageTag { get; set; }
     }
 }