title: Doctrine return null in place of EntityNotFoundException
body: <p>I have broken FKs in my database and if I load an entity and ask for a related entity Doctrine will throw <code>\Doctrine\ORM\EntityNotFoundException</code>.</p>
  
  <p>For the entity in question, I would prefer that where the FK is broken it would return NULL rather than throw an exception. This is because its within a Twig template that the exception occurs and I would prefer Twig to not have to have to handle the exception in this case.</p>
  
  <p>The following is an example configuration.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
                    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                    xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd"&gt;
      &lt;entity name="Foo\Click" table="clicks"&gt;
          &lt;id name="id" type="bigint" column="click_id"&gt;
              &lt;generator strategy="IDENTITY"/&gt;
          &lt;/id&gt;
          &lt;!-- .. --&gt;
          &lt;many-to-one field="visitor" target-entity="Foo\Visitor" fetch="LAZY"&gt;
              &lt;join-columns&gt;
                  &lt;join-column name="visitor_id" referenced-column-name="visitor_id"/&gt;
              &lt;/join-columns&gt;
          &lt;/many-to-one&gt;
      &lt;/entity&gt;
  
      &lt;entity name="Foo\Visitor" table="visitors" read-only="true"&gt;
          &lt;id name="visitorId" type="integer" column="visitor_id"&gt;
              &lt;generator strategy="IDENTITY"/&gt;
          &lt;/id&gt;
          &lt;!-- ... --&gt;
          &lt;one-to-one field="firstClick" target-entity="Foo\Click" fetch="LAZY"&gt;
              &lt;join-columns&gt;
                  &lt;join-column name="click_id" referenced-column-name="click_id"/&gt;
              &lt;/join-columns&gt;
          &lt;/one-to-one&gt;
      &lt;/entity&gt;
  &lt;/doctrine-mapping&gt;
  </code></pre>
  
  <p>The following is an example of expected results where the the click as a visitor ID, but the a visitor record does not exists with that ID. In this case, I would rather not have to wrap the logic in Try/Catch and instead have <code>Click::getVisitor()</code> return <code>null</code>;</p>
  
  <pre><code>&lt;?php
  $clickOne = $entityManager()-&gt;find(Foo\Click::class, 1);
  $v = $clickOne-&gt;getVisitor();
  
  if ($v !== null) {
      echo $v-&gt;getId(); // may throw Doctrine\ORM\EntityNotFoundException
  }
  </code></pre>
  
  <p>Is there a strategy for this with Doctrine?</p>
  
  <hr>
  
  <p><strong>Update:</strong> Added example configuration and code, and now I see the why this is not achievable with a simple Doctrine configuration.</p>