title: fillna(0) changing other values
body: <p>This is the frequency_creative dataframe</p>
  
  <pre><code>[('tags', 'int'), 
   ('user_id', 'bigint'), 
   ('processdate', 'date'), 
   ('brandsurvey_name', 'string'), 
   ('survey_id', 'string'), 
   ('questionid', 'string'), 
   ('questiontext', 'string'), 
   ('frequency', 'bigint'), 
   ('creative_id', 'string')]
  </code></pre>
  
  <p>When i use <code>fillna(0)</code> then some values of user_id column is getting corrupted.But,if <code>fillna(0)</code> is not used the things are working fine.</p>
  
  <pre><code>frequency_creative.select("user_id").distinct().show()
  
   +-------------------+
   |            user_id|
   +-------------------+
   |1665009053012894694|
   | 840031193618494976|
   +-------------------+
  
  
  frequency_creative  =frequency_creative.select("processdate","tags","survey_id","brandsurvey_name","user_id","questionid","questiontext","frequency","creative_id").fillna(0)
  frequency_creative.select("user_id").distinct().show()
  
  after select
  +-------------------+
  |            user_id|
  +-------------------+
  |1665009053012894720|
  | 840031193618494976|
  +-------------------+
  **********************************************
  without fillna(0)
  
  
  before select
  +-------------------+
  |            user_id|
  +-------------------+
  |1665009053012894694|
  | 840031193618494976|
  +-------------------+
  
  frequency_creative  =frequency_creative.select("processdate","tags","survey_id","brandsurvey_name","user_id","questionid","questiontext","frequency","creative_id")
  frequency_creative.select("user_id").distinct().show()
  
  after select
  +-------------------+
  |            user_id|
  +-------------------+
  |1665009053012894694|
  | 840031193618494976|
  +-------------------+
  </code></pre>