title: Filter pyspark dataframe to keep rows containing at least 1 null value (keep, not drop)
body: <p>Suppose I have the following pyspark dataframe:</p>
  
  <pre><code>&gt;&gt;&gt; df = spark.createDataFrame([('A', 'Amsterdam', 3.4), ('B', 'London', None), ('C', None, None), ('D', None, 11.1)], ['c1', 'c2', 'c3'])
  &gt;&gt;&gt; df.show()
  +---+---------+----+
  | c1|       c2|  c3|
  +---+---------+----+
  |  A|Amsterdam| 3.4|
  |  B|   London|null|
  |  C|     null|null|
  |  D|     null|11.1|
  +---+---------+----+
  </code></pre>
  
  <p>How can I now select or filter for <strong>any</strong> row, containing <strong>at least one</strong> null value, like so?:</p>
  
  <pre><code>&gt;&gt;&gt; df.SOME-COMMAND-HERE.show()
  +---+---------+----+
  | c1|       c2|  c3|
  +---+---------+----+
  |  B|   London|null|
  |  C|     null|null|
  |  D|     null|11.1|
  +---+---------+----+
  </code></pre>