How to deal with a NULL in Connect By Level without the query going on forever?

I have a leases table, with start and end date of each lease. Some leases don't have an end date, and the field is NULL. I want to generate a list of the payment dates/amounts over the duration of the lease. For the leases with an end date, I'm using CONNECT BY LEVEL \<= TRUNC(MONTHS_BETWEEN(pl.END_DATE, pl.START_DATE)+1,0).

However if I try to handle the case when the end_date is NULL, then the query goes on forever.

Sample data, lease 2 works fine and generates 13 rows, while lease 4 just goes on forever.

I've tried various iterations of replacing the NULL with the current date, as an output field and in the results.

SELECT  CASE WHEN rownum = 1 THEN pl.START_DATE ELSE ADD_MONTHS(trunc(pl.START_DATE, 'mon') + pl.PAYMENT_DAY -1, rownum -1) END AS PAYMENT_DATE
         , pl.*
         , rownum AS pm_num
FROM    PM_LEASES pl  
WHERE   pl.LEASE_ID = 4
CONNECT BY LEVEL <= TRUNC(MONTHS_BETWEEN(nvl(pl.END_DATE, trunc(sysdate)), pl.START_DATE)+1,0)

Also tried reading the Oracle documentation, however it focuses on actual hierarchies.I'm using this as a workaround for the moment, which works if the lease is a 100 payments or less

SELECT  pl.lease_id
        , CASE WHEN lvl = 1 THEN pl.first_payment_amt ELSE pl.lease_amt END AS lease_amt
        , CASE WHEN lvl = 1 THEN pl.start_date ELSE ADD_MONTHS(trunc(pl.START_DATE, 'mon') + pl.PAYMENT_DAY -1, lvl-1) END AS payment_date
FROM    pm_leases pl
LEFT JOIN (select level as lvl from dual connect by level <= 100) ON 1=1
WHERE   add_months(pl.start_date, lvl) <= nvl(pl.end_date, add_months(trunc(sysdate), 1))

From Oracle 12, you can LATERAL join to a row generator and use COALESCE(end_date, SYSDATE) to get the final day:

SELECT  CASE m.months
        WHEN 0
        THEN pl.start_date
        ELSE ADD_MONTHS(
               TRUNC(pl.start_date, 'MM') + pl.payment_date - 1,
               m.months
             )
        END AS PAYMENT_DATE,
        pl.*
FROM    PM_LEASES pl
        CROSS JOIN LATERAL(
          SELECT LEVEL - 1 AS months
          FROM   DUAL
          CONNECT BY LEVEL <= MONTHS_BETWEEN(
                                COALESCE(pl.end_date, SYSDATE),
                                TRUNC(pl.start_date, 'MM') + pl.payment_date - 1
                              ) + 1
        ) m
-- WHERE   pl.LEASE_ID = 4

Which, for the sample data:

CREATE TABLE pm_leases (Lease_Id, Payment_date, Start_Date, End_Date) AS
SELECT 4, 12, DATE '2023-07-16', NULL FROM DUAL UNION ALL
SELECT 2, 28, DATE '2024-05-20', DATE '2025-06-14' FROM DUAL;

Outputs:

or, depending on how you want to handle payment dates that may occur in the same month as the starting month:

SELECT  GREATEST(
          ADD_MONTHS(
            TRUNC(pl.start_date - pl.payment_date + 1, 'MM')
            + pl.payment_date - 1,
            m.months
          ),
          pl.start_date
        ) AS PAYMENT_DATE,
        pl.*
FROM    PM_LEASES pl
        CROSS JOIN LATERAL(
          SELECT LEVEL - 1 AS months
          FROM   DUAL
          CONNECT BY ADD_MONTHS(
                       TRUNC(pl.start_date - pl.payment_date + 1, 'MM')
                       + pl.payment_date - 1,
                       LEVEL - 1
                    ) <= COALESCE(pl.end_date, SYSDATE)
        ) m
--WHERE   pl.LEASE_ID = 4

Which outputs:

Note: You may need something more robust if payment_date is set to 29-31 as not all months have that many days.

fiddle