Boolean values in config are ignored in Admin UI if they are false

[jacobwod]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Go to Admin UI -> LayerSwitcher config
2. Set visibleAtStart to false. This saves visibleAtStart: false to your map config.
3. Open up Client. LayerSwitcher won't show. So far so good.
4. Go back to Admin UI. Reload map config. The checkbox is still toggled.

Why?
Because:

visibleAtStart: existingConfig.visibleAtStart || this.state.visibleAtStart,

grabs the false in the first step and then the || operator sees false which makes it to continue to the next variable, so it reads the value from existing state (where it is true).

This should've been:

visibleAtStart: existingConfig.visibleAtStart ?? this.state.visibleAtStart,