Add element with name and atts to etree iff there are any atts.

        Parameters:
            etree - an etree object
            name  - XML element name
            atts  - dicts of attribute values. Attribute names are transformed

def add_element_with_atts_to_etree(self, etree, name, atts):
        """Add element with name and atts to etree iff there are any atts.

        Parameters:
            etree - an etree object
            name  - XML element name
            atts  - dicts of attribute values. Attribute names are transformed
        """
        xml_atts = {}
        for att in atts.keys():
            val = atts[att]
            if (val is not None):
                xml_atts[self._xml_att_name(att)] = str(val)
        if (len(xml_atts) > 0):
            e = Element(name, xml_atts)
            if (self.pretty_xml):
                e.tail = "\n"
            etree.append(e)