rts: Fix SET_HDR initialization of retainer set

This fixes a regression in retainer set profiling introduced by
b0293f78. Prior to that commit
the heap traversal word would be initialized by `SET_HDR` using
`LDV_RECORD_CREATE`. However, the commit added a `doingLDVProfiling`
check in `LDV_RECORD_CREATE`, meaning that this initialization no longer
happened.

Given that this initialization was awkwardly indirectly anyways, I have
fixed this by explicitly initializating the heap traversal word to
`NULL` in `SET_PROF_HDR`. This is equivalent to the previous behavior,
but much more direct.

Fixes #24513.