Change VhdUri to Mandatory since this is required.

Author: hyonholee

src/ResourceManager/Compute/Commands.Compute/VirtualMachine/Config/AddAzureVMDataDiskCommand.cs

@@ -47,7 +47,7 @@ public class AddAzureVMDataDiskCommand : Microsoft.Azure.Commands.ResourceManage
         public string Name { get; set; }
 
         [Parameter(
-            Mandatory = false,
+            Mandatory = true,
             Position = 2,
             ValueFromPipelineByPropertyName = true,
             HelpMessage = HelpMessages.VMDataDiskVhdUri)]
@@ -115,8 +115,8 @@ protected override void ProcessRecord()
                 Name = this.Name,
                 Caching = this.Caching,
                 DiskSizeGB = this.DiskSizeInGB,
-                Lun = this.Lun == null ? 0 : this.Lun.Value,
-                VirtualHardDisk = string.IsNullOrEmpty(this.VhdUri) ? null : new VirtualHardDisk
+                Lun = this.Lun.GetValueOrDefault(),
+                VirtualHardDisk = new VirtualHardDisk
                 {
                     Uri = this.VhdUri
                 },