title: Need Help In Converting cuDF Dataframe to cupy ndarray
body: <p>I want to convert a cuDF dataframe to cupy ndarray.
  I'm using this code below:</p>
  
  <pre><code>import time
  import numpy as np
  import cupy as cp
  import cudf
  from numba import cuda
  df = cudf.read_csv('titanic.csv')
  arr_cupy = cp.fromDlpack(df.to_dlpack())
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-176-0d6ff9785189&gt; in &lt;module&gt;
  ----&gt; 1 arr_cupy = cp.fromDlpack(df.to_dlpack())
  
  ~/.conda/envs/rapids_013/lib/python3.7/site-packages/cudf/core/dataframe.py in to_dlpack(self)
     3821         import cudf.io.dlpack as dlpack
     3822 
  -&gt; 3823         return dlpack.to_dlpack(self)
     3824 
     3825     @ioutils.doc_to_csv()
  
  ~/.conda/envs/rapids_013/lib/python3.7/site-packages/cudf/io/dlpack.py in to_dlpack(cudf_obj)
       72         )
       73 
  ---&gt; 74     return libdlpack.to_dlpack(gdf_cols)
  
  cudf/_libxx/dlpack.pyx in cudf._libxx.dlpack.to_dlpack()
  
  ValueError: Cannot create a DLPack tensor with null values.                     Input is required to have null count as zero.
  </code></pre>
  
  <p>I'm getting this error because dataset have nullvalues.
  How can I do this??</p>