title: Set doctrine entity boolean field to 0 instead of null
body: <p>Im trying to persist an doctrine entity with a boolean field where the values are 0 or 1.</p>
  
  <p>When the property is set to true, it save it as '1' in database.
  But when its 'false' or '0', it save it as NULL on database.</p>
  
  <p>How can I fix this to only save only as 1 or 0 ?</p>
  
  <p>The annotation for the property I use is like following:</p>
  
  <pre><code>@ORM\Column(name="substitute", type="boolean", nullable=true)
  </code></pre>
  
  <p>When I set nullable to false, I cant persist it because it still want to set to null.</p>
  
  <p>Thanks</p>
  
  <p>When I persist it, the field value is 0</p>
  
  <p><strong>Attempt 1</strong>
  @ORM\Column(name="substitute", type="boolean", options={"default":"0"}))</p>
  
  <p><em>error: Can't save null</em></p>
  
  <p><strong>Attempt 2</strong>
  @ORM\Column(name="substitute", type="boolean", nullable= true, options={"default":"0"}))</p>
  
  <p><em>Doesn"t work, it still save null in base</em></p>
  
  <p><strong>Info 1</strong></p>
  
  <p><em>The actually insert query is trying to insert 0. But I got this error "ORA-01400: cannot insert NULL into (\"MYBASE\".\"MYTABLE\".\"SUBSTITUTE\")"</em></p>
  
  <p><strong>Info 2</strong></p>
  
  <p>Same append with another entity</p>
  
  <pre><code>class TestEntity
  {
      /**
       * @ORM\Column(name="test_entity_id", type="integer")
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      private $id;
  
      /**
       * @ORM\Column(name="substitute", type="boolean")
       */
      private $isSubstitute = false;
  }
  </code></pre>
  
  <p>Persisting</p>
  
  <pre><code>$test = new TestEntity();
  $test-&gt;setIsSubstitute(false);
  $em-&gt;persist($test);
  </code></pre>
  
  <p>Result</p>
  
  <pre><code>request.CRITICAL: Uncaught PHP Exception Doctrine\DBAL\Exception\NotNullConstraintViolationException: "An exception occurred while executing 'INSERT INTO TestEntity (test_entity_id, substitute) VALUES (?, ?)' with params [7, 0]: SQLSTATE[HY000]: General error: 1400 OCIStmtExecute: ORA-01400: cannot insert NULL into ("MYBASE"."TESTENTITY"."SUBSTITUTE")  (ext\pdo_oci\oci_statement.c:148)"\n (ext\\pdo_oci\\oci_statement.c:148) at PATH\\vendor\\doctrine\\dbal\\lib\\Doctrine\\DBAL\\Driver\\PDOStatement.php:91)"} []
  </code></pre>
  
  <p><strong>Info 3</strong></p>
  
  <p><em>Inserting manually works using oci or oci8 driver</em></p>
  
  <pre><code>sql&gt; INSERT INTO TestEntity (test_entity_id, substitute) VALUES (13, 0)
  [2017-04-06 11:21:15] 1 row affected in 62ms
  </code></pre>