Fix backward_hermite_coefficients to work in the case where ys=[]. Ad…

…d unit test case to test_interpolation_classes

diffrax/_global_interpolation.py

@@ -14,6 +14,7 @@
     from typing import ClassVar as AbstractVar
 else:
     from equinox import AbstractVar
+
 from equinox.internal import ω
 from jaxtyping import Array, ArrayLike, PyTree, Real, Shaped
 
@@ -760,7 +761,7 @@ def backward_hermite_coefficients(
     fn = ft.partial(_backward_hermite_coefficients, fill_forward_nans_at_end, ts)
 
     if len(jtu.tree_leaves(ys)) == 0:
-        return ({}, {}, {}, {})
+        return tuple(ys for _ in range(4))
 
     if deriv0 is None:
         if replace_nans_at_start is None:

test/test_global_interpolation.py

@@ -258,11 +258,7 @@ 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: jr.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_: