This change causes the GetCertificate callback to be called if
Certificates is empty. Previously this configuration would result in an
error.

This allows people to have servers that depend entirely on dynamic
certificate selection, even when the client doesn't send SNI.

Fixes #9208.

Change-Id: I2f5a5551215958b88b154c64a114590300dfc461
Reviewed-on: https://go-review.googlesource.com/8792Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>