The source-layer field is optional, but why can't pass undefined

[frylstar]

The source-layer field is optional, but when I pass the value undefined, the console gives an error: Error: layers.outline.source-layer: string expected, undefined found. I'm confused about this sign, but it doesn't seem to report an error in previous versions.
I provide an example of adding source-layer : undefined below.

mapbox-gl-js version: v3.7.0

browser: Chrome

Link to Demonstration

https://jsbin.com/zepefoq/edit?html,output

Expected Behavior

[layer.sourceLayer
string?](https://docs.mapbox.com/mapbox-gl-js/api/map/#map#addlayer)

"source-layer"?: string,