Allow empty publish type argument in push.ps1

push.ps1

@@ -2,7 +2,7 @@
 #requires -version 4
 [cmdletbinding(SupportsShouldProcess = $true)]
 param(
-    [ValidateSet('default', 'blob')]
+    [ValidateSet('default', 'blob', '')]
     [string]$PublishType = 'default',
 
     # used for PublishType = default
@@ -19,6 +19,11 @@ param(
 $ErrorActionPreference = 'Stop'
 Set-StrictMode -Version 2
 
+if (-not $PublishType) {
+    Write-Warning "Skipping push because `$PublishType was empty"
+    exit 0
+}
+
 if ($env:BUILD_IS_PERSONAL) {
     $WhatIfPreference = $true
     Write-Host -ForegroundColor Yellow 'Automatically setting -WhatIf for personal builds'