Since commit b1c3ae33 (cmTarget: Short-circuit language computation if
context independent., 2014-04-09, v3.1.0-rc1~669^2~1) 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