title: Empty String is not treated as null in Hive
body: <p>My understanding of the following statement is that if blank or empty string is inserted into hive column, it will be treated as null.</p>
  
  <pre><code>TBLPROPERTIES('serialization.null.format'=''
  </code></pre>
  
  <p>To test the functionality i have created a table and insertted '' to the filed 3. When i query for nulls on the field3, there are no rows with that criteria.</p>
  
  <p>Is my understanding of making blank string to null correct??</p>
  
  <pre><code>CREATE TABLE CDR
  (
  field1                 string,
  field2                 string,
  field3                 string
  )
  ROW FORMAT DELIMITED FIELDS TERMINATED BY ',' LINES TERMINATED BY '\n'
  **TBLPROPERTIES('serialization.null.format'='');**
  
  insert overwrite table emmtest.cdr select **field1,field2,''** from emmtest.cdr_non_orc;
  
  select * from emmtest.cdr where **field3 is null;**
  </code></pre>
  
  <p>The last statement has not returned any rows. But i am expecting all rows to be returned since there is blank string in field3.</p>