fix(backends/fstar): no __marker_trait if parent bounds

[W95Psp]

Parent bounds are translated as record fields in F*.  Also, F* classes
are records, and F* records cannot be empty. Thus, we insert a dummy
field `__marker_trait` on empty traits (marker traits).

On trait definitions, we were inserting this `__marker_trait` fields
only when a trait had no parent bounds and no associated item (method,
assoc. type, assoc fun.).

On instance definitons, we were inserting `__marker_trait` when the
trait was empty, disregarding parent bounds.

This difference of treatments lead to TC issues in F*.

This PR fixes this discrepancy.