Only inject XSD documentation annotations processed from JavaDoc if the JavaDoc is non-empty

[lennartj]

Currently, the XsdAnnotationProcessor always injects XSD documentation annotations into generated XSD documents.

However, that frequently yields a lot of empty documentation nodes which makes the generated schema unnecessarily difficult to read. Thus, improve the XsdAnnotationProcessor to only inject XSD documentation nodes when required.

<xs:annotation>
            <xs:documentation></xs:documentation>
</xs:annotation>

[lennartj]

Fixed in eadf40a16afb60b094bd0de7adeedbd4aecdf4bc