fix(legacy-json): misc promotion logic fix

Re nodejs/node#57343 (comment)

Signed-off-by: flakey5 <[email protected]>

Author: flakey5

src/generators/legacy-json/index.mjs

@@ -67,7 +67,7 @@ export default {
         if (output) {
           await writeFile(
             join(output, `${node.api}.json`),
-            JSON.stringify(section)
+            JSON.stringify(section, null, 2)
           );
         }
       })

src/generators/legacy-json/utils/buildSection.mjs

@@ -1,3 +1,4 @@
+// @ts-check
 import { buildHierarchy } from './buildHierarchy.mjs';
 import { parseList } from './parseList.mjs';
 import { enforceArray } from '../../../utils/array.mjs';
@@ -116,11 +117,11 @@ export const createSectionBuilder = () => {
         // Only promote certain keys
         if (!UNPROMOTED_KEYS.includes(key)) {
           // Merge the section's properties into the parent section
-          parent[key] = parent[key]
-            ? // If the parent already has this key, concatenate the values
-              [].concat(parent[key], value)
-            : // Otherwise, directly assign the section's value to the parent
-              [];
+          if (parent[key] && Array.isArray(parent[key])) {
+            parent[key] = parent[key].concat(value);
+          } else {
+            parent[key] ||= value;
+          }
         }
       });
     }