BaseTools:change some incorrect parameter defaults

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=1858

for Dict={},There are pitfalls in the way this default parameter is set
and Dict is not used in functions, other functions have these two cases,
I will change some incorrect parameter defaults

This patch is going to fix this issue

Cc: Liming Gao <liming.gao@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/GenFds/OptRomFileStatement.py

@@ -34,7 +34,10 @@ class OptRomFileStatement:
     #   @param  Dict        dictionary contains macro and value pair
     #   @retval string      Generated FFS file name
     #
-    def GenFfs(self, Dict = {}, IsMakefile=False):
+    def GenFfs(self, Dict = None, IsMakefile=False):
+
+        if Dict is None:
+            Dict = {}
 
         if self.FileName is not None:
             self.FileName = GenFdsGlobalVariable.ReplaceWorkspaceMacro(self.FileName)