crypto/internal/boring: tolerate empty ECDSA signatures

crypto/internal/boring: tolerate empty ECDSA signatures

VerifyASN1 became directly reachable without encoding/decoding in
CL 353849, so it's now possible for the signature to be empty.

Change-Id: I37d6400945ab541120180bf73335e0ec93322947
Reviewed-on: https://go-review.googlesource.com/c/go/+/452635
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>