Make MaxActiveSessions not nullable

Fixes a bad assumption with the previous migration.

Jellyfin.Server.Implementations/Migrations/20201004171403_AddMaxActiveSessions.cs

@@ -13,7 +13,7 @@ namespace Jellyfin.Server.Implementations.Migrations
                 name: "MaxActiveSessions",
                 schema: "jellyfin",
                 table: "Users",
-                nullable: true);
+                nullable: false);
         }
 
         protected override void Down(MigrationBuilder migrationBuilder)

Jellyfin.Server.Implementations/Migrations/JellyfinDbModelSnapshot.cs

@@ -344,7 +344,7 @@ namespace Jellyfin.Server.Implementations.Migrations
                     b.Property<int?>("LoginAttemptsBeforeLockout")
                         .HasColumnType("INTEGER");
 
-                    b.Property<int?>("MaxActiveSessions")
+                    b.Property<int>("MaxActiveSessions")
                         .HasColumnType("INTEGER");
 
                     b.Property<int?>("MaxParentalAgeRating")