title: Convert empty number as null
body: <p>is there a possibility to tell my JSF-implementation to convert empty numbers as null. 
  So far I always get 0 (of the corresponging complex-type).<br>
  For empty strings there is <strong>javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL</strong>, but I couldn't find anything for numbers.  By the way: No, I can't use the vm-argument: <strong>-Dorg.apache.el.parser.COERCE_TO_ZERO=false</strong></p>
  
  <p>Is a custom-numberconverter my only possibility?</p>