title: Count of rows containing null values in pyspark
body: <p>Consider a pyspark dataframe for example</p>
  
  <pre><code>columns = ['id', 'dogs', 'cats']
  vals = [(1, 2, 0),(None, 0, 1),(5,None,9)]
  df=spark.createDataFrame(vals,columns)
  df.show()
  +----+----+----+
  |  id|dogs|cats|
  +----+----+----+
  |   1|   2|   0|
  |null|   0|   1|
  |   5|null|   9|
  +----+----+----+
  </code></pre>
  
  <p>I want to write a code which returns 2 as the number of rows containing null values</p>