cmake: Don't assert that imported zephyr libraries have source files

A Zephyr library not having source files is usually due to a
misconfiguration and will lead to an obscure error message. But
imported Zephyr libraries don't have source files so we need to skip
the source file when a library is imported.

An imported Zephyr library is normally used to reference externally
built code, either prebuilt or recursively built through an external
build system.

This fixes #5497

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>