Profiler should not miss query's top-level access paths nodes

[asfernandes]

For example, when explaining this query:

SQL> select 1, (select 2 from rdb$database b) from rdb$database a;

Sub-query
    -> Singularity Check
        -> Table "RDB$DATABASE" as "B" Full Scan
Select Expression
    -> Table "RDB$DATABASE" as "A" Full Scan

The profiler plg$prof_record_sources table misses nodes for Sub-query and Select Expression.

[pavel-zotov]

Issue is covered by test for #7687