BUG #19736: `JSON_OBJECT ... ABSENT ON NULL` accepts invalid keys when the value is NULL

The following bug has been logged on the website:

Bug reference: 19736
Logged by: Shallow
Email address: theshallow27(at)gmail(dot)com
PostgreSQL version: 18.6
Operating system: Linux
Description:

When `ABSENT ON NULL` removes a pair because its value is SQL NULL,
`JSON_OBJECT` also skips validation of that pair's key. The documented
restrictions say that a key cannot be NULL and must be a scalar expression
without a cast to JSON. Those restrictions should still apply when the value
pair will be omitted.

**Reproduction:**

```sql
SELECT JSON_OBJECT((NULL::text) VALUE NULL::text
ABSENT ON NULL RETURNING json);

SELECT JSON_OBJECT((ARRAY[1,2]) VALUE NULL::text
ABSENT ON NULL RETURNING json);
```

**Actual result:** Both statements return `{}`.

**Expected result:** Both statements should reject the invalid key, as the
key
restrictions apply regardless of whether the value is omitted.