cmake: Fail with an error message when empty libraries exist

A very annoying usability issue is that the error message is very
cryptic when you create a zephyr library that doesn't have any source
files. Creating such a library is very easy to do, so we should have a
good error message for it.

It was also considered to allow empty libraries to exist, but this was
decided against as they show up in the build output and can confuse
the end user.

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