empty `startup_session` breaks `--title`

[luxzeitlos]

Describe the bug
When the startup_session file is empty, then running kitty --title FOO does not set the title of the new window to FOO.

if --single-instance is passed and an instance is already open it works

To Reproduce
Steps to reproduce the behavior:

1. set startup_session startup_session in the kitty.conf
2. ensure startup_session is an empty file
3. run kitty --title FOO
4. See the window title is not FOO

Expected behavior
The window title should be FOO

Environment details
OS: NixOS 20.09.git.7badbf18c45 (Nightingale)
shell: fish

Output of kitty --debug-config

kitty 0.18.3 created by Kovid Goyal
Linux lux-plane 5.4.69 #1-NixOS SMP Thu Oct 1 11:19:27 UTC 2020 x86_64
<<< Welcome to NixOS 20.09.git.7badbf18c45 (\m) - \l >>>

Run 'nixos-help' for the NixOS manual.
Loaded config files: /home/lux/.config/kitty/kitty.conf
Running under: X11

Config options different from defaults:
font_family     Source Sans Pro
font_size       13.0
shell           fish
startup_session /home/lux/.config/kitty/startup_session