Change OxideQSecurityStatus::certificate() and OxideQSslCertificate::issuer() to return a QVariant

[Chris Coulson]

Bug 1453294 converted OxideQSslCertificate to a value type. As a consequence, where OxideQSecurityStatus::certificate() and OxideQSslCertificate::issuer() used to return nullptr they now return a value where OxideQSslCertificate::isValid() returns false.

This has a couple of issues:
- It's a slight change in API behaviour. Any applications that are walking the issuer chain and checking for null to detect the top of the chain will now end up in an infinite loop.
- It's weird from QML to have to check SslCertificate.isValid rather than null.

It might be appropriate instead to have these accessors return QVariants. This will allow us to keep OxideQSslCertificate as a value type whilst being able to represent a null certificate.