Fix assertion when a calculator does not return anything (#308)

rascaline/src/calculator.rs

@@ -56,7 +56,9 @@ impl<'a> LabelsSelection<'a> {
               G: FnOnce(&Labels) -> Result<Vec<Labels>, Error>,
               H: Fn(TensorBlockRef<'_>) -> Labels,
     {
-        assert_ne!(keys.count(), 0);
+        if keys.count() == 0 {
+            return Ok(vec![]);
+        }
 
         match *self {
             LabelsSelection::All => {
@@ -294,6 +296,7 @@ impl Calculator {
     #[time_graph::instrument(name="Calculator::prepare")]
     fn prepare(&mut self, systems: &mut [Box<dyn System>], options: CalculationOptions) -> Result<TensorMap, Error> {
         let default_keys = self.implementation.keys(systems)?;
+
         let keys = match options.selected_keys {
             Some(keys) if keys.is_empty() => {
                 return Err(Error::InvalidParameter("selected keys can not be empty".into()));

rascaline/src/calculators/neighbor_list.rs

@@ -938,4 +938,21 @@ mod tests {
         ));
 
     }
+    #[test]
+    fn check_empty_response() {
+        let mut calculator = Calculator::from(Box::new(NeighborList {
+            cutoff: 0.1,
+            full_neighbor_list: true,
+            self_pairs: false,
+        }) as Box<dyn CalculatorBase>);
+        let mut systems = test_systems(&["water"]);
+
+        let descriptor = calculator.compute(&mut systems, Default::default()).unwrap();
+
+        // cutoff too low, TensorMap is empty!
+        assert_eq!(descriptor.keys(), &Labels::new::<i32,2>(
+            ["first_atom_type", "second_atom_type"],
+            &[],
+        ));
+    }
 }