Add default values for the site and case prefix parameters
Add default values for the site and case prefix parameters

In @mperignon's original, these parameters default to `None`, which
are translated to `null` in JSON, which apparently causes Javascript (?)
to silently abort. I don't think this issue should go back to the model
developer, so I've handled it here by giving these parameters
default values that are strings.

csdms/wmt-metadata