fixes#24631
[Object
constructors](793baf34ff/compiler/semobjconstr.nim (L462)),
[casts](793baf34ff/compiler/semexprs.nim (L494))
and [type
conversions](793baf34ff/compiler/semexprs.nim (L419))
copy their type nodes verbatim instead of producing semchecked type
nodes. This causes a crash in transf when an untyped expression in the
type node has `nil` type. To deal with this, don't try to transform the
type node in these expressions at all. I couldn't reproduce the problem
with type conversion nodes though so those are unchanged in transf.
(cherry picked from commit 6d59680217)
