title: data frame is_null returning wrong result
body: <p>I am using a python templating engine and saving the rendered code to run_cmd.py. I run this python script through a subprocess. In this template I load from a csv file to pandas dataframe. From subprocess I get the dataframe as a string. I now want to check if the dataframe that was loaded have missing values or not. So, I read the string using read_csv using StringIO. Now the dataframe looks like this:</p>
  
  <pre><code>           Name       City Country  Years
  0  0        abc          X      US      1
  1  1       abcd          Y      US      5
  2  2       abde          Z      US    NaN
  </code></pre>
  
  <p>On checking for null values on this dataframe, I get False which is not correct. Any ideas what is going wrong?</p>
  
  <pre><code>code =  env.get_template('load_df_from_csv.template').render(path = 'xyz.csv')
  with open("run_cmd.py", "w") as fh:
      fh.write(code)
      fh.close()
  temp = subprocess.check_output(['python', './run_cmd.py'])
  str_data = StringIO(temp)
  df = pd.read_csv(str_data, index_col = False)
  missing_value = df.isnull().values.any()
  </code></pre>