When Lektor encounters a content file it needs to decide which data model to
use for figuring out how to handle the fields. In the ideal case the content
_model field which will point explicitly to a model. However
if a model was not defined it's picked automatically.
Lektor will attempt to load a few models and will pick the first that exists.
If no model exists at all then Lektor calls back to the special
which is empty other than for system fields.
nonemodel which is always empty.