BUG: sparse: coo_matrix.todia() would fail when the matrix was all zeros.

This PR fixes a bug in coo_matrix.todia().  The method raises an exception if the array is all zeros.  For example:

    In [3]: from scipy.sparse import coo_matrix

    In [4]: z = [[0, 0]]

    In [5]: c = coo_matrix(z)

    In [6]: d = c.todia()
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-6-6294988ff569> in <module>()
    ----> 1 d = c.todia()

    /home/warren/anaconda/lib/python2.7/site-packages/scipy/sparse/coo.pyc in todia(self)
        336 
        337         #initialize and fill in data array
    --> 338         data = np.zeros( (len(diags), self.col.max()+1), dtype=self.dtype)
        339         data[ np.searchsorted(diags,ks), self.col ] = self.data
        340 

    /home/warren/anaconda/lib/python2.7/site-packages/numpy/core/_methods.pyc in _amax(a, axis, out, keepdims)
          8 def _amax(a, axis=None, out=None, keepdims=False):
          9     return um.maximum.reduce(a, axis=axis,
    ---> 10                             out=out, keepdims=keepdims)
         11 
         12 def _amin(a, axis=None, out=None, keepdims=False):

    ValueError: zero-size array to reduction operation maximum which has no identity