strip/objcopy: handle empty file as unknown

Previously strip reported a somewhat cryptic error for empty files:

strip: elf_begin() failed: Invalid argument

Add a special case to treat empty files as with an unknown file format.
This is consistent with llvm-strip. GNU strip produces no output which
does not seem like useful behaviour (but it does exit with status 1).

Reported by: andrew
Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32648