conf: fix title and description for virDomainSetMetadata API

conf: fix title and description for virDomainSetMetadata API

If we pass XML to virDomainDefineXML API with these two elements:

    ...
    <title></title>
    <description></description>
    ...

libvirt correctly ignores these two elements and they will not appear
in the parsed XML.

However, if we use virDomainSetMetadata API and with "" as value for
title or description we will end up with the parsed XML that contains
these empty elements.

Let's fix the behavior of this API to behave the same as
virDomainDefineXML.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1518042

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>