title: Null bitmap is set when there is no null columns
body: <p>I am learning PAGE structure and currently I'm stuck at the NULL bitmaps.</p>
  
  <pre><code>create table dbo.ro
  (
  ID int not null,
  Col1 varchar(8000) null,
  Col2 varchar(8000) null
  );
  insert into dbo.ro(ID, Col1, Col2) values
  (1,replicate('a',8000),replicate('b',8000));
  </code></pre>
  
  <p>So currently there is no NULL values, let's see DBCC info:</p>
  
  <pre><code>DBCC IND(test, 'ro', 1);
  DBCC PAGE('test',1, 408,3);
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vN3Pp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vN3Pp.png" alt="enter image description here"></a>
  So I am interested in the following part
  30000800 01000000 03005002</p>
  
  <ul>
  <li>30 - BitsA</li>
  <li>00 - BitsB</li>
  <li>0800 - Fdata length</li>
  <li>01000000 - Fixed data (ID = 1)</li>
  <li>0300 - number of columns</li>
  <li>50 - NULL bitmap</li>
  </ul>
  
  <p>Why is it 50 and not 00? There is no NULL values in the record ...</p>