Don't send 'Tab' when 'autotype' and 'username' is undefined

[stonewareslord]

Here: https://github.com/carnager/rofi-pass/blob/master/rofi-pass#L45, it looks like tab will always be sent if a password has no autotype field, but I think that rofi should detect if a username field exists, and only send a tab if it does.

Here's a use case that works as expected from the current implementation. router-password:

password

---
user: username

rofi-pass would send username\tpassword through xdotool, but if router-password contains:

password

---

rofi-pass would just send \tpassword, which would almost always tab out of the highlighted password field. I believe a good solution is detecting if USERNAME_field is defined, if so, send ${USERNAME_field}\t${password}, if not, just send ${password}