Flag an error on empty rust module srcs

It's an error if a rust module's srcs is empty. Prior to this change
the output was a panic that complains loudly about a slice error and
nothing else. This change doesn't stop the panic, but at least adds
a bit of context so devs who make a simple mistake can more easily
fix it.

Test: SOONG_GEN_RUST_PROJECT=1 m nothing
Change-Id: Id7d8465d533413c3000699661222a53a7c8678f3