Make the toBlob() callback argument non-nullable

As implemented in Gecko, it's not nullable, so this throws:
document.createElement('canvas').toBlob(null)

Passing in null also doesn't make sense, as with no callback you have
no way of getting the Blob, so it must be a programmer error.

source

@@ -60645,7 +60645,7 @@ interface <dfn>HTMLCanvasElement</dfn> : <span>HTMLElement</span> {
   <span>CanvasProxy</span> <span data-x="dom-canvas-transferControlToProxy">transferControlToProxy</span>();
 
   DOMString <span data-x="dom-canvas-toDataURL">toDataURL</span>(optional DOMString type, any... arguments);
-  void <span data-x="dom-canvas-toBlob">toBlob</span>(<span>FileCallback</span>? _callback, optional DOMString type, any... arguments);
+  void <span data-x="dom-canvas-toBlob">toBlob</span>(<span>FileCallback</span> _callback, optional DOMString type, any... arguments);
 };</pre>
    </dd>
   </dl><!--REMOVE-TOPIC:DOM APIs-->
@@ -61097,8 +61097,6 @@ interface <dfn>HTMLCanvasElement</dfn> : <span>HTMLElement</span> {
 
    <li><p>Return, but continue running these steps <span>in parallel</span>.</p></li>
 
-   <li><p>If <var>callback</var> is null, abort these steps.</p></li>
-
    <li><p><span>Queue a task</span> to invoke the <code>FileCallback</code> <var>callback</var> with
    <var>result</var> as its argument. The <span>task source</span> for this task is the <dfn>canvas
    blob serialisation task source</dfn>.</p></li>