title: Return list of single element if stream filter condition doesn't return any results
body: <p>I'm currently filtering a stream but if the filter doesn't return any matches I'd like to return a single default value. This is within a chain of additional streams so I'm using this to avoid the chain stopping if one step doesn't have any results.</p>
  
  <p>Currently I'm fudging it by collecting the result of the filter into a list, if the list is empty, create my new default list and return it as a stream. If the list isn't empty, convert the result back into a stream to pass it back.</p>
  
  <p>What is a more stream orented way to achive this without needing to go to lists and back to streams?</p>