Validate expiration field

Validation should be added to the "Password Expiration" field. If a user leaves this field blank the following error is returned and all settings for the new policy are lost.

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'expiration' at row 1: INSERT INTO {password_policy} (name, description, enabled, policy, expiration, warning) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5); Array ( [:db_insert_placeholder_0] => Test [:db_insert_placeholder_1] => this will break [:db_insert_placeholder_2] => 0 [:db_insert_placeholder_3] => a:0:{} [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => ) in password_policy_admin_form_submit() (line 444 of C:\inetpub\wwwroot\drupal\sites\all\modules\password_policy\password_policy.admin