Fix per-config sources in multi-config generators when first config adds none

Since commit b1c3ae33 we've tried to avoid repeating computation of the list of sources for a target for every configuration in the case that a per-config source (or object library) contributes zero sources. However, it is possible that an entry contributes zero sources in the first configuration processed but at least one source in other configurations.

Fixes: #25400 (closed)