Merge pull request Azure#620 from huangpf/release-0.9.5

Remove ValidateNotNullOrEmpty for DiskSizeInGB (for the disk attaching use case)

Author: markcowl

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

@@ -68,7 +68,6 @@ public class AddAzureVMDataDiskCommand : AzurePSCmdlet
             Position = 4,
             ValueFromPipelineByPropertyName = true,
             HelpMessage = HelpMessages.VMDataDiskSizeInGB)]
-        [ValidateNotNullOrEmpty]
         public int? DiskSizeInGB { get; set; }
 
         [Parameter(