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.