Fix "text preparation algorithm" to account for NaN

The behavior was previously undefined when calling, for example,
ctx.fillText(text, x, y, NaN) or ctx.fillText(text, x, y, undefined).

Fixes #1452.

source

@@ -60856,8 +60856,8 @@ transform. ack Shaun Morris. -->
 
   <ol>
 
-   <li><p>If <var>maxWidth</var> was provided but is less than or equal to zero, return an
-   empty array.</p></li>
+   <li><p>If <var>maxWidth</var> was provided but is less than or equal to zero or equal to NaN,
+   return an empty array.</p></li>
 
    <li><p>Replace all the <span data-x="space character">space characters</span> in <var>text</var> with U+0020 SPACE characters.</p></li>