Make ServiceName optional for verify-changelog

In cases like go where we don't pass a service name having this default to "not-specified" breaks things so we should allow for ServiceName and/or ServiceDirectory to be empty.

eng/common/pipelines/templates/steps/verify-changelog.yml

@@ -4,7 +4,7 @@ parameters:
   default: 'not-specified'
 - name: ServiceName
   type: string
-  default: 'not-specified'
+  default: ''
 - name: ServiceDirectory
   type: string
   default: ''
@@ -23,4 +23,4 @@ steps:
       pwsh: true
       workingDirectory: $(Pipeline.Workspace)
     displayName: Verify ChangeLogEntry for ${{ parameters.PackageName }}
-    continueOnError: false
+    continueOnError: false