BaseTools: Structure Pcd value override incorrect.

This patch is going to fix the issue that
The Pcd field value is override incorrectly when there
is no Pcd overall value assignment in Dsc file.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/Workspace/BuildClassObject.py

@@ -61,7 +61,7 @@ class PcdClassObject(object):
         self.validlists = validlists
         self.expressions = expressions
         self.DscDefaultValue = None
-        self.DscRawValue = None
+        self.DscRawValue = {}
         if IsDsc:
             self.DscDefaultValue = Value
         self.PcdValueFromComm = ""