[BUG] JSQLParser 5.3 : PostgreSQL : COMMENT ON ... IS NULL (comment removal) fails to parse #2562

Failing SQL Feature:

COMMENT ON <object> IS NULL cannot be parsed. In PostgreSQL, writing NULL in place of the text string is the way to remove a comment — per the COMMENT docs: "To remove a comment, write NULL in place of the text string." So a DDL stream that drops comments cannot be processed.

SQL Example:

COMMENT ON TABLE app.t IS NULL

fails on 5.3 and on current master (6c726d8) with:

net.sf.jsqlparser.parser.ParseException: Encountered: <K_NULL> / "NULL", at line 1, column 27, in lexical state DEFAULT.
Was expecting this terminal within expansion starting at 15161:20:
    <S_CHAR_LITERAL> ...

COMMENT ON COLUMN app.t.name IS NULL and COMMENT ON VIEW app.v IS NULL fail the same way. All string-literal forms parse fine, including the empty string (IS 'hello', IS '').

Software Information:

• JSqlParser 5.3 (current Maven Central release); reproduced on master 6c726d8
• PostgreSQL 18 (verified: COMMENT ON TABLE t IS NULL removes the comment, i.e. deletes the pg_description row; IS '' behaves the same)

I'll follow up with a PR (grammar + round-trip tests).