Fix CubicInterpolation for the case where ys = {}. Add unit tests for both cubic and linear interpolation checking this case

Author: allen-adastra

diffrax/_global_interpolation.py

@@ -758,6 +758,10 @@ def backward_hermite_coefficients(
 
     ts = _check_ts(ts)
     fn = ft.partial(_backward_hermite_coefficients, fill_forward_nans_at_end, ts)
+
+    if len(jtu.tree_leaves(ys)) == 0:
+        return ({}, {}, {}, {})
+
     if deriv0 is None:
         if replace_nans_at_start is None:
             coeffs = jtu.tree_map(fn, ys)

test/test_global_interpolation.py

@@ -261,6 +261,7 @@ def test_interpolation_classes(mode, getkey):
     ys_ = [
         _make(),
         [_make(), {"a": _make(), "b": _make()}],
+        {}
     ]
     for ts in ts_:
         assert len(ts) == length