XSLDocumentTest should test for empty transform stylesheet

[simonjenga]

final XSL xsl = new XSLDocument(
    StringUtils.join(
        "<xsl:stylesheet  ",
        " xmlns:xsl='http://www.w3.org/1999/XSL/Transform' ",
        " version='2.0' >",
        "</xsl:stylesheet>"
    )
);

http://www.java2s.com/Code/XML/XSLT-stylesheet/Emptytransform.htm

[dmarkov]

@yegor256 take a look at this issue please and dispatch it somehow, see par.21