improve rules send email action FROM field

the 'send email' action in the drupal rules implementation has some flaws with how the FROM email address is handled.

1) the form retrieves the site's default FROM address to populate the default FROM field value. you can overwrite and save, and it works fine. but when you edit the form, it reverts to the default value, because it lacks conditions to see if a value is already set.

2) setting the FROM value to the default site value should also take place in the action itself – not just in the action form – if no value is set. currently, it's possible to leave an empty value in the form field, which results in SMTP errors when the rule action is triggered from the interface.

the attached patch addresses both issues.