xmute whitespace generates empty expression

The xmute mutator whitespace (REPLACE) generates an empty expression.

This is a mutation from paymentmeans-iban-tests.xml:

 <cac:PaymentMeans>
    
      <cbc:PaymentMeansCode>58</cbc:PaymentMeansCode>
      <cac:PayeeFinancialAccount>
      <?xmute mutator="code" values="DE 75512108001245126199, DE75 51 2108 00124512 6199" separator="," schematron-valid="BR-DE-19" id="iban-with-whitespace-inside" description="Iban whitespace inside, should still be validated"?>
      <!--This is the active mutation configuration: paymentmeans-iban-tests.xml-61-REPLACE -->
<?xmute mutator="whitespace" schematron-valid="BR-DE-19" id="iban-with-whitespace" description="Iban with whitespace, should still be validated"?>
         <cbc:ID> 	 

</cbc:ID>
      </cac:PayeeFinancialAccount>
  </cac:PaymentMeans>