Fedora StorageAdapter finds root node for `nil` identifer

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adapter = ::Valkyrie::Storage::Fedora.new(connection: my_connection))
adapter.find_by(id: nil)

gives the RDF for the root node:

#<Valkyrie::StorageAdapter::StreamFile id=nil io=#<Valkyrie::Storage::Fedora::IOProxy:0x00007fbef52d2380 @source="@prefix premis:  <http://www.loc.gov/premis/rdf/v1#> .\n@prefix test:  <info:fedora/test/> .\n@prefix rdfs:  <http://www.w3.org/...

it should instead raise an error. likely this applies to any id that doesn't match an ldp:NonRDFSource, which is kinda neat, but probably not what we want.