openconnect: Make FSID passphrase + empty private key combination work.

It should be possible to usen FSID-protected key passphrase with a user
certificate and no private key.

This was not working as expected because we were always calling
openconnect_set_client_cert() with QByteArray::data(). The latter will pass
an empty string rather than nullptr if it is empty, which can be the case
for `key` if it is not set. This causes openconnect_set_client_cert() to use
that empty string rather than handle the null argument case, and we would be
unexpectedly prompted for a key password when trying to connect.

Avoid running into this by explicitly passing `nullptr` when we have an
empty `key`.

BUG: 443770
1 job for master in 1 minute and 20 seconds (queued for 5 seconds)