WMTS parsing bugfix backport

Backport of fix geopython/OWSLib@d2d7c60 to fix geopython/OWSLib#361

Author: tomkralidis

python/ext-libs/owslib/wmts.py

@@ -130,7 +130,8 @@ def _buildMetadata(self, parse_remote_metadata=False):
 
         #serviceProvider metadata
         serviceprov=self._capabilities.find('{http://www.opengis.net/ows/1.1}ServiceProvider')
-        self.provider=ServiceProvider(serviceprov)
+        if serviceprov is not None:
+            self.provider = ServiceProvider(serviceprov)
 
         #serviceOperations metadata
         self.operations=[]