title: How to concatenate to a null column in pyspark dataframe
body: <p>I have a below dataframe and I wanted to update the rows dynamically with some values</p>
  
  <pre><code>input_frame.show()
  +----------+----------+---------+
  |student_id|name      |timestamp|
  +----------+----------+---------+
  |        s1|testuser  |       t1|
  |        s1|sampleuser|       t2|
  |        s2|test123   |       t1|
  |        s2|sample123 |       t2|
  +----------+----------+---------+
  
  input_frame = input_frame.withColumn('test', sf.lit(None))
  input_frame.show()
  +----------+----------+---------+----+
  |student_id|      name|timestamp|test|
  +----------+----------+---------+----+
  |        s1|  testuser|       t1|null|
  |        s1|sampleuser|       t2|null|
  |        s2|   test123|       t1|null|
  |        s2| sample123|       t2|null|
  +----------+----------+---------+----+
  
  input_frame = input_frame.withColumn('test', sf.concat(sf.col('test'),sf.lit('test')))
  input_frame.show()
  +----------+----------+---------+----+
  |student_id|      name|timestamp|test|
  +----------+----------+---------+----+
  |        s1|  testuser|       t1|null|
  |        s1|sampleuser|       t2|null|
  |        s2|   test123|       t1|null|
  |        s2| sample123|       t2|null|
  +----------+----------+---------+----+
  </code></pre>
  
  <p>I want to update the 'test' column with some values and apply the filter with partial matches on the column. But concatenating to null column resulting in a null column again. How can we do this?</p>