Check for DocsMsPackagesAllValid before running validation step (#4916)

Co-authored-by: Daniel Jurek <djurek@microsoft.com>

eng/common/pipelines/templates/steps/docsms-ensure-validation.yml

@@ -1,6 +1,6 @@
 steps:
 # Fail the build if any of the packages failed validation. Valid values are
-# "true" or "false"
+# "true" or "false". This step will skip if $(DocsMsPackagesAllValid) is not set
 - pwsh: |
     if ('$(DocsMsPackagesAllValid)' -eq 'true') {
       Write-Host "All packages passed validation."
@@ -9,3 +9,4 @@ steps:
       exit 1
     }
   displayName: Check package validation results
+  condition: and(succeeded(), ne(variables['DocsMsPackagesAllValid'], ''))