Ensure that space lat and long can be cleared by setting blank strings

Author: rsmithlal

app/models/better_together/geography/space.rb

@@ -25,11 +25,11 @@ def geocoded?
       end
 
       def latitude= arg
-        super if arg.present?
+        super(arg.presence)
       end
 
       def longitude= arg
-        super if arg.present?
+        super(arg.presence)
       end
 
       def to_leaflet_point