Fix bug that returns attribute error when lat or long is exactly 0 (#44)

* Fix bug that returns attribute error when lat or long is exactly 0

* replace  '!= None' with  'is not None'

Author: rochacbruno

flask_googlemaps/__init__.py

@@ -98,9 +98,9 @@ def build_marker_dict(self, marker, icon=None):
         return marker_dict
 
     def add_marker(self, lat=None, lng=None, **kwargs):
-        if lat:
+        if lat is not None:
             kwargs['lat'] = lat
-        if lng:
+        if lng is not None:
             kwargs['lng'] = lng
         if 'lat' not in kwargs or 'lng' not in kwargs:
             raise AttributeError('lat and lng required')