Guard against missing context during SSR in TreeControls

`wrapRootElement` is only registered in `gatsby-browser.js`, not
`gatsby-ssr.js`, so during static HTML generation `useSkoHubContext()`
returns the raw `defaultState` object instead of `{ data, updateState }`.
That made `data` undefined at render time and broke SSR with
`Cannot read properties of undefined (reading 'sortBy')` for flat
schemes (where TreeControls now renders to host the sort selector).

Optional-chain the `data` reads in TreeControls so the component
renders harmlessly during SSR; the dropdown becomes interactive once
the browser hydrates inside the ContextProvider.

Verified with `npm run build` against the interactivityType fixture
(the exact page that was failing in CI).

Author: sroertgen

src/components/TreeControls.jsx

@@ -35,12 +35,12 @@ const TreeControls = ({ hasNesting = true }) => {
 
   // Initialise sortBy from config on first render if it hasn't been set yet.
   useEffect(() => {
-    if (data.sortBy == null) {
+    if (data && data.sortBy == null) {
       updateState({ ...data, sortBy: config?.sortBy || "prefLabel" })
     }
-  }, [data.sortBy, config?.sortBy])
+  }, [data?.sortBy, config?.sortBy])
 
-  const currentSort = data.sortBy ?? config?.sortBy ?? "prefLabel"
+  const currentSort = data?.sortBy ?? config?.sortBy ?? "prefLabel"
 
   return (
     <div className="TreeControls" css={style}>
@@ -75,7 +75,10 @@ const TreeControls = ({ hasNesting = true }) => {
         <select
           aria-label="Sort entries by"
           value={currentSort}
-          onChange={(e) => updateState({ ...data, sortBy: e.target.value })}
+          onChange={(e) =>
+            updateState &&
+            updateState({ ...(data || {}), sortBy: e.target.value })
+          }
         >
           <option value="prefLabel">Label</option>
           <option value="notation">Notation</option>