Allow using KMAC with an empty key (#55)

The requirement that the key length is non-zero was copied from HMAC,
but NIST SP 800-185 section 4.2 explicitly says:

> K is a key bit string of any length, including zero.

(with a caveat about the security strength depending on the length of
the key, of course).

So, match the underlying spec and allow using KMAC with an empty key.

Author: twiss

index.html

@@ -6352,17 +6352,9 @@ <h5>Generate Key</h5>
                 |normalizedAlgorithm| is present:
               </dt>
               <dd>
-                <ol>
-                  <li>
-                    Let |length| be equal to the
-                    {{KmacKeyGenParams/length}}
-                    member of |normalizedAlgorithm|.
-                  </li>
-                  <li>
-                    If |length| is zero, then [= exception/throw =] an
-                    {{OperationError}}.
-                  </li>
-                </ol>
+                Let |length| be equal to the
+                {{KmacKeyGenParams/length}}
+                member of |normalizedAlgorithm|.
               </dd>
               <dt>
                 Otherwise, if the {{Algorithm/name}} member of
@@ -6570,13 +6562,6 @@ <h5>Import Key</h5>
               Let |length| be the length in bits of |data|.
             </p>
           </li>
-          <li>
-            <p>
-              If |length| is zero
-              then [= exception/throw =] a
-              {{DataError}}.
-            </p>
-          </li>
           <li>
             <dl class="switch">
               <dt>