_drupal_session_write() does not always return a boolean

Follow-up of #2460833: _drupal_session_destroy() should return boolean

Problem/Motivation

When Drupal is not allowed to save the session, _drupal_session_write() returns nothing. This results into the following warnings in PHP 7:

Warning: session_write_close(): Session callback expects true/false return value
Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct

This happens in ± line 166:

if (!drupal_save_session()) {
  // We don't have anything to do if we are not allowed to save the session.
  return;
}

I found a similar situation in _drupal_session_destroy() but there TRUE is returned (± line 426):

// Nothing to do if we are not allowed to change the session.
if (!drupal_save_session()) {
  return TRUE;
}

Steps to reproduce

drupal_save_session(FALSE);
session_write_close();

For background information, see also #2677808: compatible PHP7.

Proposed resolution

I think that _drupal_session_write() should return TRUE when it is not allowed to save the session, as that also happens in _drupal_session_destroy() in a similar situation.

Patch will follow.