Make sure getters for timestamp fields return integers

Describe your bug or feature request.

Currently, all of our methods for getting the completed, placed and created timestamps can return strings (even though the documented return type is int (See the example below):

  /**
   * Gets the order placed timestamp.
   *
   * @return int
   *   The order placed timestamp.
   */
  public function getPlacedTime();

drush ev "var_dump(\Drupal\commerce_order\Entity\Order::load(120)->getPlacedTime())"
string(10) "1752501150"

Also, we need to specify that NULL can be returned and explicitly return NULL when the field is empty.

Let's also return NULL for various other fields for which we aren't explicitly returning NULL (the total price, the email and the IP address for example.