Rollup merge of #124023 - pacak:less-splody, r=jieyouxu

Allow workproducts without object files.

This pull request partially reverts changes from e16c3b4

Original motivation for this assert was described with "A WorkProduct without a saved file is useless"
which was true at the time but now it is possible to have work products with other types of files
(llvm-ir, asm, etc) and there are bugreports for this failure:

For example: rust-lang/rust#123695

Fixes rust-lang/rust#123234

Now existing `assert` and `.unwrap_or_else` are unified into a single
check that emits slightly more user friendly error message if an object
files was meant to be produced but it's missing

Author: GuillaumeGomez