Fix FutureWarning in extract_xml_list.
Fix FutureWarning in extract_xml_list.

Checking `if elements` raises a `FutureWarning`, but it's not necessary
as the comprehensions will produce an empty list for empty input anyway.

kalxas/OWSLib,ocefpaf/OWSLib,tomkralidis/OWSLib,bird-house/OWSLib,geopython/OWSLib