fix #12 (issues adding VM to empty DrsVMGroup)

DRSRule.psm1

@@ -1084,8 +1084,8 @@ function Set-DrsVMGroup {
 
         ## if all VMs were specified to be remove from the group (if the $groupSpec.Info.VM value is $null), write warning and take no further action (may not be supported by vSphere API, seemingly; trying to do so via GUI returns message to the effect of, "Cannot remove all members of group")
         if ($null -eq $groupSpec.Info.VM) {Write-Warning "Removing all VMs from VMGroup not supported. Taking no action"}
-        ## if the VM list is the same between the existing VMGroup and the new ClusterGroupSpec, do not bother calling ReconfigureComputeResource() method
-        elseif ($null -eq (Compare-Object -ReferenceObject $arrOriginalVMIDsInTarget -DifferenceObject $groupSpec.Info.VM)) {
+        ## if the VMGroup had any VMs in it already, and the VM list is the same between the existing VMGroup and the new ClusterGroupSpec, do not bother calling ReconfigureComputeResource() method
+        elseif (($null -ne $arrOriginalVMIDsInTarget) -and $null -eq (Compare-Object -ReferenceObject $arrOriginalVMIDsInTarget -DifferenceObject $groupSpec.Info.VM)) {
             Write-Verbose "Not changing VMGroup (no new members added, and no members to remove)"
         } ## end if
         else {

changelog.md

@@ -4,6 +4,7 @@
 
 ### v1.2.1
 Apr 2018
+- \[bugfix] fixed [Issue #12](https://github.com/PowerCLIGoodies/DRSRule/issues/12), "Error at addition in empty group the VM list" -- can now add a VM to a DrsVMGroup that has no VM members to start, for whatever reason (VM atrophy, maybe?)
 - \[bugfix] fixed [Issue #15](https://github.com/PowerCLIGoodies/DRSRule/issues/15), "Metadata file VMware.Vim.dll could not be found" -- now loads items properly for VMware PowerCLI version 10 as well, and on Windows PowerShell and PowerShell Core.
 
 ### v1.2.0

todo.md

@@ -15,3 +15,4 @@
 - update `Get-Drs*` behavior to match standard `Get-*` cmdlets' behavior of returning error when no item found of given name when `$Name` has no wildcard:
 	- `Get-DrsVMGroup bogusGroupName` should throw error
 - add function to get objects from exported JSON, to essentially return info items that could be used for `New-Drs*`, but that would be useful to see what items are stored in given JSON
+- maybe (if only for the sake of thoroughness):  look into addressing potential issues that might arise from `Compare-Object` calls where a reference- or difference object might be $null (which would cause `Compare-Object` to throw an error) -- an example of where this was fixed:  `Set-DrsVMGroup`; may be minimal chance of hitting bug, as DRS VM/VMHost groups should generally have members, but may be worth a look throughout rest of code