Fix FutureWarning when truth-testing XML element

[vuilleumierc]

Instantiating a WebMapTileService currently raises a warning:

>>> from owslib.wmts import WebMapTileService
>>> wmts = WebMapTileService("https://wmts.geo.admin.ch/EPSG/4326/1.0.0/WMTSCapabilities.xml")
/tmp/.venv/lib/python3.10/site-packages/owslib/wmts.py:862: FutureWarning: Truth-testing of elements was a source of confusion and will always return True in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.
  if current and current.text == 'true':

see also #979

Thanks in advance for considering this PR :)

[geographika]

Looks good to me. Thanks @vuilleumierc.