fix: Stop the automatic upgrade from discarding valid configuration (#2692)

- Closes #2689

tests/config/schema/common/preprocess-to-array.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, it } from 'vitest';
+import { z } from 'zod';
+
+import { preprocessToArray } from '../../../../src/config/schema/common/preprocess-to-array';
+
+const schema = z.object({
+  items: preprocessToArray(z.object({ name: z.string() }).array()).optional(),
+});
+
+describe('preprocessToArray', () => {
+  it('should keep a list as it is', () => {
+    expect(schema.parse({ items: [{ name: 'office' }] })).toEqual({
+      items: [{ name: 'office' }],
+    });
+  });
+
+  it('should normalise a single item to a list', () => {
+    expect(schema.parse({ items: { name: 'office' } })).toEqual({
+      items: [{ name: 'office' }],
+    });
+  });
+
+  it('should read a key written with no value as a list of nothing', () => {
+    expect(schema.parse({ items: null })).toEqual({ items: [] });
+  });
+
+  it('should leave an absent optional field absent', () => {
+    expect(schema.parse({})).toEqual({});
+  });
+});