Steps to reproduce:

PostgreSQL Agent 2 has 2 items that have missing credentials in the key parameters.

Result:

• Item PostgreSQL: Cache hit become Not supported failing with: Connection failed: failed to connect to `host=localhost user=postgres database=postgres`: failed SASL auth (FATAL: password authentication failed for user "postgres" (SQLSTATE 28P01)).
• Item PostgreSQL: Age of oldest xid become Not supported failing with: Connection failed: failed to connect to `host=localhost user=zbx_monitor database=postgres`: failed SASL auth (FATAL: password authentication failed for user "zbx_monitor" (SQLSTATE 28P01)).

Expected:

Both items should return values without failing.

To solve just be sure the macros PG.USER and PG.PASSWORD are present on both item's keys like the one enabled in following screenshot: