Global state value becomes `undefined` when not set but other keys are set

**Describe the bug** When using the `["global-state", key]` expression with a key that has not been set yet, and if any other keys _have_ been set, the value is resolved as `undefined`. **To Reproduce** I made a tiny reproduction at https://github.com/Alduino/maplibre-global-state-reprod, you can run it with `pnpm run dev`. It just sets the text colour to red if `["==", ["global-state", "test"], null]` or blue otherwise. 1. Text initially is red; global state object is empty 2. Click `Set "unrelated" to true` which sets a different global state key to a value; text becomes blue due to the resolved value for `test` becoming `undefined` 3. Click `Set "test" to null` to explicitly set the test global state value to null; text becomes red again. Or if it's easier to read pseudocode: ```js evaluate(["==", ["global-state", "test"], null]); // null map.setGlobalState("unrelated", true); evaluate(["==", ["global-state", "test"], null]); // undefined(!) map.setGlobalState("test", null); evaluate(["==", ["global-state", "test"], null]); // back to null ``` **Expected behavior** As [described in the docs](https://maplibre.org/maplibre-style-spec/expressions/#global-state) the global state expression should return `null` if the value isn't set. **Screenshots** | | | | |---|---|---| | <img width="600" height="735" alt="Image" src="https://github.com/user-attachments/assets/d2676a84-3e06-49af-a852-ccd58cd87eb5" /> | <img width="600" height="784" alt="Image" src="https://github.com/user-attachments/assets/be92f7af-0010-4068-bead-7c301905d7b3" /> | <img width="600" height="810" alt="Image" src="https://github.com/user-attachments/assets/ee54466d-9343-4c40-8a60-0751e5c11764" /> | **Desktop (please complete the following information):** - OS: Fedora 43 KDE Plasma Edition - Browser: Firefox 148 - Version: maplibre-gl v5.19.0, @maplibre/maplibre-gl-style-spec 24.6.0 **Smartphone (please complete the following information):** - N/A **Additional context** Not sure if this should be posted here or in the maplibre-gl-js repo, issue seems to be from the code in this repo though. As far as I can tell it's coming from here: - https://github.com/maplibre/maplibre-style-spec/blob/8d85feb97dbe27112f4504ee6317d3e0b55bfed7/src/expression/definitions/global_state.ts#L38-L44 - https://github.com/maplibre/maplibre-style-spec/blob/8d85feb97dbe27112f4504ee6317d3e0b55bfed7/src/util/get_own.ts#L13-L15 If the global state object isn't defined or has no keys then `null` is returned, otherwise `getOwn` returns `undefined` instead of `null`. Maybe `evaluate` could return `getOwn() ?? null`? That would also return `null` even if the value is explicitly set to `undefined`, although seeing as afaict in the style spec `undefined` basically doesn't exist maybe that would make sense too?