Do not render None as the value for an empty custom vars dictionary

src/pypiwik/tracker.py

@@ -163,6 +163,8 @@ class PiwikTracker(object):
 
 			if property_name in ('page_custom_vars', 'visit_custom_vars'):
 				value = self._build_cvars(value)
+				if not value:
+					continue
 
 			d[parameter_name] = value