fix(db-postgres): in query with null (#12661)

Previously, this was possible in MongoDB but not in Postgres/SQLite
(having `null` in an `in` query)
```
const { docs } = await payload.find({
  collection: 'posts',
  where: { text: { in: ['text-1', 'text-3', null] } },
})
```
This PR fixes that behavior

test/database/int.spec.ts

@@ -2619,4 +2619,33 @@ describe('database', () => {
     expect(res.testBlocks[0]?.text).toBe('text')
     expect(res.testBlocksLocalized[0]?.text).toBe('text-localized')
   })
+
+  it('should support in with null', async () => {
+    await payload.delete({ collection: 'posts', where: {} })
+    const post_1 = await payload.create({
+      collection: 'posts',
+      data: { title: 'a', text: 'text-1' },
+    })
+    const post_2 = await payload.create({
+      collection: 'posts',
+      data: { title: 'a', text: 'text-2' },
+    })
+    const post_3 = await payload.create({
+      collection: 'posts',
+      data: { title: 'a', text: 'text-3' },
+    })
+    const post_null = await payload.create({
+      collection: 'posts',
+      data: { title: 'a', text: null },
+    })
+
+    const { docs } = await payload.find({
+      collection: 'posts',
+      where: { text: { in: ['text-1', 'text-3', null] } },
+    })
+    expect(docs).toHaveLength(3)
+    expect(docs[0].id).toBe(post_null.id)
+    expect(docs[1].id).toBe(post_3.id)
+    expect(docs[2].id).toBe(post_1.id)
+  })
 })