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/GenFdsGlobalVariable.py

@@ -742,7 +742,7 @@ class GenFdsGlobalVariable:
     #   @param  MacroDict     Dictionary that contains macro value pair
     #
     @staticmethod
-    def MacroExtend (Str, MacroDict={}, Arch=DataType.TAB_COMMON):
+    def MacroExtend (Str, MacroDict=None, Arch=DataType.TAB_COMMON):
         if Str is None:
             return None