Throw when length == 0 in the ECDH derive bits operation

Author: twiss

spec/Overview.html

@@ -8714,6 +8714,11 @@ <h4>Operations</h4>
             <dt>Derive Bits</dt>
             <dd>
               <ol>
+                <li>
+                  <p>
+                    If |length| is zero, then [= exception/throw =] an {{OperationError}}.
+                  </p>
+                </li>
                 <li>
                   <p>
                     If the {{CryptoKey/[[type]]}} internal slot of