You need to look up the pg_locks table for those entries with an empty pid value. Run this query:

SELECT t.schemaname || '.' || t.relname AS tablename,
       l.pid, l.granted
       FROM pg_locks l JOIN pg_stat_user_tables t
       ON l.relation = t.relid;

The output will be something similar to the following:

 tablename |  pid  | granted
-----------+-------+---------
    db.x   |       | t
    db.x   | 27289 | f
(2 rows)

The preceding example shows a lock on the db.x table, which has no process associated with it.

If you need to remove a particular prepared transaction, you can refer to the Removing old prepared transactions recipe in Chapter 9, Regular Maintenance.