Avoid passing null to valid name prefix

Tests: web-platform-tests/wpt#53533.

Fixes #1387.

Author: shannonbooth

dom.bs

@@ -245,10 +245,12 @@ U+0000 NULL, or U+003E (>).
    <li><p>Set <var>prefix</var> to <var>splitResult</var>[0].
 
    <li><p>Set <var>localName</var> to <var>splitResult</var>[1].
+
+   <li><p>If <var>prefix</var> is not a [=valid namespace prefix=], then [=throw=] an
+   "{{InvalidCharacterError}}" {{DOMException}}.
   </ol>
 
- <li><p>If <var>prefix</var> is not a [=valid namespace prefix=], then [=throw=] an
- "{{InvalidCharacterError}}" {{DOMException}}.
+ <li><p><a for=/>Assert</a>: <var>prefix</var> is either null or a [=valid namespace prefix=].
 
  <li><p>If <var>context</var> is "<code>attribute</code>" and <var>localName</var> is not a
  [=valid attribute local name=], then [=throw=] an "{{InvalidCharacterError}}" {{DOMException}}.