crypto/tls: fix ServerHello SCT test

According to https://tools.ietf.org/html/rfc6962#section-3.3, the SCT
must be at least one byte long. The parsing code correctly checks for
this condition, but rarely the test does generate an empty SCT.

Change-Id: If36a34985b4470a5a9f96affc159195c04f6bfad
Reviewed-on: https://go-review.googlesource.com/c/129755


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>