Improve parsing of Length Units in javax/swing/text/html/CSS

The parsing of length units in javax/swing/texthtml/CSS takes an unnecessary path when a percentage is parsed.

That is, when a String value like "100%" is encountered, it is correctly parsed as a percentage value to begin with. But then the control flow of the method does not return but rather runs through the handling of point values, encounters an NFE, swallows it and returns. This can be improved.