vterm: Fixed a crash bug with DECALN command

[danschwarz]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

vterm.py was crashing upon execution of DECALN (ESC # 8) because the code was sending a string to empty_line which requires a byte. This prevented vttest from running many test cases.

to test this, run terminal.py and execute the following shell command:

echo -e "\033#8"

Without this fix, urwid crashes. With the fix, you'll see a lot of Es onscreen, as expected. No crash.