title: GROUP_CONCAT return NULL if any value is NULL
body: <p>How can I make <code>GROUPT_CONCAT</code> return <code>NULL</code> if any column is <code>NULL</code>?</p>
  
  <p>Here is a test table:</p>
  
  <pre><code>CREATE TABLE gc (
    a INT(11) NOT NULL,
    b VARCHAR(1) DEFAULT NULL
  );
  
  INSERT INTO gc (a, b) VALUES
  (1, 'a'),
  (1, 'b'),
  (2, 'c'),
  (2, NULL),
  (3, 'e');
  </code></pre>
  
  <p>And my query:</p>
  
  <pre><code>SELECT a, GROUP_CONCAT(b)
  FROM gc
  GROUP BY a;
  </code></pre>
  
  <p>This is what I get:</p>
  
  <pre><code>a | GROUP_CONCAT(b)
  --+----------------
  1 | a,b
  2 | c
  3 | e
  </code></pre>
  
  <p>This is what I want:</p>
  
  <pre><code>a | GROUP_CONCAT(b)
  --+----------------
  1 | a,b
  2 | NULL
  3 | e
  </code></pre>