Is there a way I can expand a query if one value is null in Oracle SQL?

Is there a way that I can expand an existing table into something like a view, replacing the (null) value with multiple records? Basically, (null) represents that the user has access to all the roles within a group.

For example, can I combine the following two tables:

Into the following table view:

Is there any way that this could be possible?

Join the two tables together on the group and on the role, if it is not null, or ignoring the role otherwise:

CREATE VIEW users_groups (username, grp, role) AS
SELECT u.username,
       u.grp,
       g.role
FROM   users u
       INNER JOIN groups g
       ON (u.grp = g.grp
           AND (u.role = g.role OR u.role IS NULL)
          )

Which, for the sample data:

CREATE TABLE users (username, grp, role) AS
SELECT 'example1', 'ABC', 100  FROM DUAL UNION ALL
SELECT 'example1', 'XYZ', 200  FROM DUAL UNION ALL
SELECT 'example2', 'ABC', NULL FROM DUAL;

CREATE TABLE groups (grp, role) AS
SELECT 'ABC', 100 FROM DUAL UNION ALL
SELECT 'ABC', 150 FROM DUAL UNION ALL
SELECT 'ABC', 200 FROM DUAL UNION ALL
SELECT 'ABC', 250 FROM DUAL UNION ALL
SELECT 'ABC', 300 FROM DUAL UNION ALL
SELECT 'XYZ', 200 FROM DUAL UNION ALL
SELECT 'XYZ', 400 FROM DUAL;

Then SELECTing from the view outputs:

fiddle