Best way to output a node even if null? (foreach, choose does not output node as expected)

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How can I output a node as null if the foreach/choose condition is not met?  Here's the source:

<appt>
   <responses>
      <answer>12312312312</answer>
      <questionExternalID>alternatephonenumber</questionExternalID>
   </responses>
   <responses>
      <answer/>
      <questionExternalID>createdbyuser</questionExternalID>
   </responses> 
</appt>

 

I am attempting to map to this structure when a condition is met:

<response-wrapper>
   <alternatenumber>081111111</alternatenumber>
</response-wrapper>

.. and this structure when it is not met:

<response-wrapper>
   <alternatenumber/>
</response-wrapper>

The problem is that the response node is repeating so I must use a foreach loop.  If I use an otherwise condition the alternatenumber node is output for each of the response nodes present, which is not what I want (I only want 1 alternatenumber node)