From UPGRADE-2.6.md:

• The "empty_value" option in the types "choice", "date", "datetime" and "time"
  was deprecated and replaced by a new option "placeholder". You should use
  the option "placeholder" together with the view variables "placeholder" and
  "placeholder_in_choices" now.

  The option "empty_value" and the view variables "empty_value" and
  "empty_value_in_choices" will be removed in Symfony 3.0.

  Before:

  $form->add('category', 'choice', array(
      'choices' => array('politics', 'media'),
      'empty_value' => 'Select a category...',
  ));

  After:

  $form->add('category', 'choice', array(
      'choices' => array('politics', 'media'),
      'placeholder' => 'Select a category...',
  ));

  Before:

  {{ form.vars.empty_value }}
  
  {% if form.vars.empty_value_in_choices %}
      ...
  {% endif %}
  

  After:

  {{ form.vars.placeholder }}
  
  {% if form.vars.placeholder_in_choices %}
      ...
  {% endif %}
  

There is no deprecation notice triggered when still using this option in 2.7.