Transforms like 'scale' need some way to handle missing data

In[92]: df = pd.DataFrame([(1,3),(2,6),(4,2),(6,5),(7,3),(4,6),(2,2),(6,4)], columns=['y','X'])
In[93]: pt.dmatrices('y ~ X.diff()', df)
Out[93]: 
(DesignMatrix with shape (7, 1)
   y
   2
   4
   6
   7
   4
   2
   6
   Terms:
     'y' (column 0),
 DesignMatrix with shape (7, 2)
   Intercept  X.diff()
           1         3
           1        -4
           1         3
           1        -2
           1         3
           1        -4
           1         2
   Terms:
     'Intercept' (column 0)
     'X.diff()' (column 1))

In[94]: pt.dmatrices('y ~ scale(X.diff())', df)

Traceback (most recent call last):
  File "C:\Python34\lib\site-packages\IPython\core\formatters.py", line 222, in catch_format_error
    r = method(self, *args, **kwargs)
  File "C:\Python34\lib\site-packages\IPython\core\formatters.py", line 699, in __call__
    printer.pretty(obj)
  File "C:\Python34\lib\site-packages\IPython\lib\pretty.py", line 368, in pretty
    return self.type_pprinters[cls](obj, self, cycle)
  File "C:\Python34\lib\site-packages\IPython\lib\pretty.py", line 552, in inner
    p.pretty(x)
  File "C:\Python34\lib\site-packages\IPython\lib\pretty.py", line 382, in pretty
    return meth(obj, self, cycle)
  File "C:\Python34\lib\site-packages\patsy\design_info.py", line 1089, in _repr_pretty_
    for col in formatted_cols]
  File "C:\Python34\lib\site-packages\patsy\design_info.py", line 1089, in <listcomp>
    for col in formatted_cols]
ValueError: max() arg is an empty sequence
Out[94]: 

The issue provides a reproducible pandas DataFrame and pt.dmatrices example; start by running the scale(X.diff()) case and comparing it with X.diff(). Trace how missing values from the transform are handled, with completion defined as a non-crashing result for this expression.