Crash when fallback to default font size using remote control

[soifou]

Describe the bug
I have no clue why this happen but, since the release of version 0.27, I am unable to fallback to default font size using remote control kitty @. The equivalent mapping key works fine though, ie map kitty_mod+backspace remote_control set-font-size 0.

To Reproduce
Steps to reproduce the behavior:

1. kitty -c NONE -o font_size=12 -o allow_remote_control=yes
2. kitty @ set-font-size -- +2 // ok
3. kitty @ set-font-size -- -1 // ok
4. kitty @ set-font-size -- 0 // crash

Traceback (most recent call last):
  File "kitty/boss.py", line 682, in _execute_remote_command
  File "kitty/remote_control.py", line 220, in handle_cmd
  File "kitty/rc/set_font_size.py", line 46, in response_from_kitty
  File "kitty/boss.py", line 1164, in change_font_size
  File "kitty/boss.py", line 1145, in calc_new_size
TypeError: '<' not supported between instances of 'float' and 'NoneType'

Error: '<' not supported between instances of 'float' and 'NoneType'

I take this opportunity to say thank you for making kitty!