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

@@ -750,6 +750,10 @@ 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

@@ -262,10 +262,7 @@ def test_interpolation_classes(mode, getkey):
         jnp.array([0.0, 2.0, 3.0, 3.1, 4.0, 4.1, 5.0, 5.1]),
     ]
     _make = lambda: jrandom.normal(getkey(), (length, num_channels))
-    ys_ = [
-        _make(),
-        [_make(), {"a": _make(), "b": _make()}],
-    ]
+    ys_ = [_make(), [_make(), {"a": _make(), "b": _make()}], {}]
     for ts in ts_:
         assert len(ts) == length
         for ys in ys_: