Don't send key_share for PSK-only key exchange

TLS 1.3 allows for the "psk_ke" and "psk_dhe_ke" key-exchange modes.
Only the latter mode introduces a new ephemeral (Diffie-Hellman)
key exchange, with the PSK being the only key material used in the
former case.

It's a compliance requirement of RFC 8446 that the server MUST NOT
send a KeyShareEntry when using the "psk_ke" mode, but prior to
this commit we would send a key-share based solely on whether the
client sent one.  This bug goes unnoticed in our internal test suite
since openssl communicating with openssl can never negotiate the
PSK-only key-exchange mode.  However, we should still be compliant
with the spec, so check whether the DHE mode was offered and don't
send a key-share if it wasn't.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14749)

(cherry picked from commit e776858b)