title: XSLT function format_number does not work for 0 with format for leading zeroes
body: <p>I would like to format a number in XSLT to have a 4 digit number with leading zeroes.</p>
  
  <p>E.g. </p>
  
  <pre><code>1234 -&gt; 1234
  12   -&gt; 0012
  1    -&gt; 0001
  0    -&gt; 0000
  </code></pre>
  
  <p>Therefor I use the function <code>format-number</code> as follows:</p>
  
  <pre><code>format-number(somevalue, '0000')
  </code></pre>
  
  <p>This works fine for everything except for 0 as <em>somevalue</em>. Here I get 0 as a result instead of 0000. </p>