title: How apply where only a column is not null in adonis query builder
body: <p>I need to check if the column <code>percentage_correct</code> >= <code>parCorrect</code> if <code>percentage_correct</code> &lt;> NULL</p>
  
  <p>I try something like:</p>
  
  <pre><code>const unidadeSequenceAtualEstudante = await Database.select("*")
              .from("student_quiz_historics")
              .where("student_id", idEstudante)
              .where(function(){
                  this
                  .whereNotNull("percentage_correct")
                  .where("percentage_correct", "&gt;=", parQtdAcerto)
              })
              .where("class_id", idClasse)
              .where("book_id", idLivro)
              .where("execution_back_status", "&lt;&gt;", "Cancelado")
              .orderBy("sequence", "desc")
              .first();
  </code></pre>
  
  <p>But when i have only records with the <code>percentage_correct</code> null, they still are trying to apply this where.</p>