wallpaper category with `monitor = ` breaks when having other categories with monitor parameter

[jonesy-b-dev]

When having this setup

wallpaper {
    monitor = 
    path = ~/.config/backgrounds/background.jpg
    fit_mode = cover
}

wallpaper {
    monitor = DP-1
    path = ~/.config/backgrounds/background.jpg
    fit_mode = cover
}

Issue

The category with monitor = does not work anymore and any other monitors remain without monitor

Expected behavior

All other remaining monitors should have the category applied with the empty monitor setting

[jmylchreest]

As per discussion on discord, I've created a quick PR for this here: #315