individual: Add default values for PresenceDetails properties

If an Individual is created with no Personas (as in the
standalone-individuals test cases), the PresenceDetails properties
should be initialised to empty strings. They were previously defaulting
to null.

https://bugzilla.gnome.org/show_bug.cgi?id=648811
individual: Allow Individual.id to be an empty string with no personas

If an Individual has no Personas (as can happen when it’s just been
constructed), document the ID as being the empty string. This is a
special case.

Previously, the ID defaulted to being null, which violated its specified
type.

https://bugzilla.gnome.org/show_bug.cgi?id=648811