Update Prepare-Release.ps1 to handle only one previous release (#1725)

In the case there is exactly one previous release PS will return the single object and thus the Count property will not exist. Instead this change ensures that we always have a list.

Co-authored-by: Wes Haggard <weshaggard@users.noreply.github.com>

eng/common/scripts/Prepare-Release.ps1

@@ -116,7 +116,7 @@ $month = $ParsedReleaseDate.ToString("MMMM")
 Write-Host "Assuming release is in $month with release date $releaseDateString" -ForegroundColor Green
 if (Test-Path "Function:GetExistingPackageVersions")
 {
-    $releasedVersions = GetExistingPackageVersions -PackageName $packageProperties.Name -GroupId $packageProperties.Group
+    $releasedVersions = @(GetExistingPackageVersions -PackageName $packageProperties.Name -GroupId $packageProperties.Group)
     if ($null -ne $releasedVersions -and $releasedVersions.Count -gt 0)
     {
       $latestReleasedVersion = $releasedVersions[$releasedVersions.Count - 1]