change None to empty string

Author: charlesdong1991

pandas/core/reshape/merge.py

@@ -489,8 +489,6 @@ def __init__(self, left, right, how='inner', on=None,
 
         self.copy = copy
 
-        if suffixes == (None, None):
-            raise ValueError('Column names cannot be identical after merging.')
         if suffixes[0] is None:
             suffixes = ('', suffixes[1])
         elif suffixes[1] is None: