Remove unnecessary if for string type

Author: kinow

pandas/core/internals/blocks.py

@@ -739,11 +739,8 @@ def to_native_types(self, slicer=None, na_rep='nan', quoting=None,
         mask = isna(values)
 
         if not self.is_object and not quoting:
-            if na_rep is not None and isinstance(na_rep, (str, bytes)):
-                itemsize = writers.max_len_string(na_rep)
-                values = values.astype("<U{size}".format(size=itemsize))
-            else:
-                values = values.astype(str)
+            itemsize = writers.max_len_string(na_rep)
+            values = values.astype("<U{size}".format(size=itemsize))
         else:
             values = np.array(values, dtype='object')