title: find NaN values is cell array
body: <p>lets assume I have the following array:</p>
  
  <pre><code>a = {1; 'abc'; NaN}
  </code></pre>
  
  <p>Now I want to find out in which indices this contains NaN, so that I can replace these with '' (empty string).</p>
  
  <p>If I use <code>cellfun</code> with <code>isnan</code> I get a useless output</p>
  
  <pre><code>cellfun(@isnan, a, 'UniformOutput', false)
  
  ans = 
  [          0]
  [1x3 logical]
  [          1]
  </code></pre>
  
  <p>So how would I do this correct?</p>