修复ctaTemplate中的发单bug（交易停止时应该返回空字符串，而不是None）

vn.trader/ctaAlgo/ctaTemplate.py

@@ -120,11 +120,16 @@ class CtaTemplate(object):
                 vtOrderID = self.ctaEngine.sendOrder(self.vtSymbol, orderType, price, volume, self) 
             return vtOrderID
         else:
-            return None        
+            # 交易停止时发单返回空字符串
+            return ''        
         
     #----------------------------------------------------------------------
     def cancelOrder(self, vtOrderID):
         """撤单"""
+        # 如果发单号为空字符串，则不进行后续操作
+        if not vtOrderID:
+            return
+        
         if STOPORDERPREFIX in vtOrderID:
             self.ctaEngine.cancelStopOrder(vtOrderID)
         else:

vn.trader/vtGateway.py

@@ -355,7 +355,7 @@ class VtContractData(VtBaseData):
     #----------------------------------------------------------------------
     def __init__(self):
         """Constructor"""
-        super(VtBaseData, self).__init__()
+        super(VtContractData, self).__init__()
         
         self.symbol = EMPTY_STRING              # 代码
         self.exchange = EMPTY_STRING            # 交易所代码