Added condition to send e-mail only when e-mail address is not empty

Main test

1. Login to adm.console
2. Create tangible product
3. Create incomplete order with this product
4. Do not fill shipping tab at all when input order data, press Save button after filling Billing tab
5. Select created order in Incomplete grid an press Place Order button in the grid's toolbar
6. Confirm, that naw order is moved to To Ship tab
7. Press Ship button in the To Ship grid's toolbar
8. Go to Logs & Reports → E-mail Log
9. Confirm that no new e-mail created with ORDER.SHIP value in the Template Name column

Regression testing

1. Repeat this test with filling e-mail and all required fields on shipping tab when creating incomplete order
2. Confirm, that in this case new ORDER.SHIP e-mail appears in the Logs & Reports → E-mail Log