title: Update-AzureRmEventGridSubscription setup subject prefix/postfix to string empty
body: <p>I have PowerShell script to update EventGrid subscriptions. One of possible scenarios is to set subscription subject prefix/postfix to define value. </p>
  
  <pre><code>Update-AzureRmEventGridSubscription -ResourceGroup $ResourceGroupName -TopicName $EventGridTopicName -EventSubscriptionName $Subscription.name -SubjectEndsWith $Subscription.subjectEndsWith
  </code></pre>
  
  <p>When value is not null or empty then it works fine. But when it needs to be set to '' it throws</p>
  
  <pre><code> Cannot validate argument on parameter 'SubjectBeginsWith'. The argument is null or empty. Provide an argument that is not null or empty, and then try the command again
  </code></pre>
  
  <p>Should I recreate subscription?</p>