Do not require address for invoice and prevent sending email without attachment

[timiwahalahti]

It was reported that an invoice email was sent without the invoice attached if the address field was left empty by bypassing the required attribute with a space character.

In this PR, we prevent the email if there's no attachment and make the address field optional.

Fixes #255

Props dd32

Screenshots

How to test the changes in this Pull Request:

1. Activate Camptix Invoices plugin and enable it on Camptix settings
2. Go to tickets page, buy a ticket with invoice
3. Address field shouldn't be required
4. Invoice should be in email
5. Go buy another ticket with invoice, but before that modify ctx_get_invoice to bail early with false value
6. No invoice email received

[timiwahalahti]

Closing in favour of #965 which combines all the Camptix Invoices changes needed to make it network activated.