title: Duplicate entry '' for key 'users_email_unique' empty field
body: <p>I have two types of users - real and fake.
  Fake users are employees, that don't use the system.
  Real users use their email address to login. So my users migration has <code>$table-&gt;string('email')-&gt;unique();</code>.</p>
  
  <p>The problem is that fake users may not have an email address. I can add first fake user no problem, but the second one generates error <code>SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'users_email_unique'</code>.</p>
  
  <p>What should I do?</p>