title: Alter table add column not null on empty table in netezza
body: <pre><code>SYSTEM.ADMIN(ADMIN)=&gt; create table test ( name varchar(20), age int);
  
  CREATE TABLE
  
  SYSTEM.ADMIN(ADMIN)=&gt; alter table test add column dob varchar(20) NOT NULL;
  </code></pre>
  
  <p>ERROR:  ALTER TABLE: not null constraint for column "DOB" not allowed without default value</p>
  
  <p>Do we have to specify a default value after not null even on empty table?</p>
  
  <pre><code>SYSTEM.ADMIN(ADMIN)=&gt; alter table test add column dob varchar(20) NOT NULL DEFAULT '0';
  ALTER TABLE
  </code></pre>
  
  <p>Is this expected behavior ?</p>