PERF: Don't build an AssetFinder if no engine is provided

This is an optimization where we're building an environment but not
using its finder.  Ideally, the consumer would use just the calendar,
but it's not fully featured quite yet.

zipline/finance/trading.py

@@ -121,7 +121,10 @@ class TradingEnvironment(object):
         else:
             self.engine = engine = asset_db_path
 
-        self.asset_finder = AssetFinder(engine)
+        if engine is not None:
+            self.asset_finder = AssetFinder(engine)
+        else:
+            self.asset_finder = None
 
     def write_data(self,
                    engine=None,