Emit empty .tasty files by default
Emit empty .tasty files by default

Before this commit, we only created .tasty files under -YemitTasty, we
haven't decided yet if this should be the default, but even if we don't
end up doing that, it is still useful to emit empty .tasty files to
signal that the corresponding .class file has a tasty section, this is
much simpler to check than parsing classfiles to look for a tasty section.

Also, fix -YemitTasty tof work with AbstractFile who are not backed by
java.io.File, previously this lead to NullPointerException.

som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty