Make `STRINGS_CTN_DECOMPOSE` an explicit conflict (#6663)

Make `STRINGS_CTN_DECOMPOSE` an explicit conflict  (#6663)

Fixes #6643. The STRINGS_CTN_DECOMPOSE inference is always a conflict
but we sometimes sent it as an inference. To make sure that the
inference manager actually recognizes the inference as a conflict, this
commit ensures that the conclusion is always false and modifies the
explanation accordingly.