shorten docstring

pandas/core/reshape/merge.py

@@ -165,8 +165,7 @@ def merge_ordered(left, right, on=None,
         `left` and `right` respectively. Pass a value of `None` instead
         of a string to indicate that the column name from `left` or
         `right` should be left as-is, with no suffix. At least one of the
-        values must not be None. A combination of `''` and `None` is will
-        raise error for columns which type is string
+        values must not be None.
     how : {'left', 'right', 'outer', 'inner'}, default 'outer'
         * left: use only keys from left frame (SQL: left outer join)
         * right: use only keys from right frame (SQL: right outer join)