E-Mail account not saved when saving thread

empty( $_POST['post_email_account'] ) was returning true
when $_POST['post_email_account'] was '0'.
Using an empty array if 'email_notifications_override' post meta array doesn't exist
Merge pull request #112 from VarunAgw/email-accounts

Check that account IDs are numeric instead of empty.

`is_empty()` returns true for 0 and '0', which led to a bug where the account ID wasn't inserted into `postmeta` when saving a thread.