Treat zero layer dataset as unusable (#4959)

* treat zero layer dataset as unusable

* update changelog

CHANGELOG.unreleased.md

@@ -11,7 +11,7 @@ For upgrade instructions, please check the [migration guide](MIGRATIONS.released
 [Commits](https://github.com/scalableminds/webknossos/compare/20.12.0...HEAD)
 
 ### Added
--
+- Datasets without any layer are now considered unimported. [#4959](https://github.com/scalableminds/webknossos/pull/4959)
 
 ### Changed
 - The menu for viewing, editing and creating annotations for a dataset in the dashboard was cleaned up a bit. Creating a hybrid (skeleton + volume) annotation is now the default way of annotating a dataset. The other options are still available via a dropdown. [#4939](https://github.com/scalableminds/webknossos/pull/4939)

app/models/binary/DataSet.scala

@@ -448,7 +448,7 @@ class DataSetDataLayerDAO @Inject()(sqlClient: SQLClient, dataSetResolutionsDAO:
     val clearQuery = sqlu"delete from webknossos.dataset_layers where _dataSet = ${_dataSet}"
 
     val queries = source.toUsable match {
-      case Some(usable) if usable.dataLayers.nonEmpty =>
+      case Some(usable) =>
         getSpecificClearQuery(usable.dataLayers) :: usable.dataLayers.map(insertLayerQuery(_dataSet, _))
       case _ => List(clearQuery)
     }

webknossos-datastore/app/com/scalableminds/webknossos/datastore/services/DataSourceService.scala

@@ -210,7 +210,8 @@ class DataSourceService @Inject()(
     if (new File(propertiesFile.toString).exists()) {
       JsonHelper.validatedJsonFromFile[DataSource](propertiesFile, path) match {
         case Full(dataSource) =>
-          dataSource.copy(id)
+          if (dataSource.dataLayers.nonEmpty) dataSource.copy(id)
+          else UnusableDataSource(id, "Error: Zero layer Dataset")
         case e =>
           UnusableDataSource(id, s"Error: Invalid json format in $propertiesFile: $e")
       }