Fix and add test for groupby_bins() isnan TypeError. (#3405)

* Fix and add test for groupby_bins() isnan TypeError.

* Better testing

* black

xarray/core/groupby.py

@@ -321,7 +321,7 @@ def __init__(
         full_index = None
 
         if bins is not None:
-            if np.isnan(bins).all():
+            if duck_array_ops.isnull(bins).all():
                 raise ValueError("All bin edges are NaN.")
             binned = pd.cut(group.values, bins, **cut_kwargs)
             new_dim_name = group.name + "_bins"

xarray/tests/test_groupby.py

@@ -276,4 +276,20 @@ def test_groupby_grouping_errors():
         dataset.to_array().groupby(dataset.foo * np.nan)
 
 
+def test_groupby_bins_timeseries():
+    ds = xr.Dataset()
+    ds["time"] = xr.DataArray(
+        pd.date_range("2010-08-01", "2010-08-15", freq="15min"), dims="time"
+    )
+    ds["val"] = xr.DataArray(np.ones(*ds["time"].shape), dims="time")
+    time_bins = pd.date_range(start="2010-08-01", end="2010-08-15", freq="24H")
+    actual = ds.groupby_bins("time", time_bins).sum()
+    expected = xr.DataArray(
+        96 * np.ones((14,)),
+        dims=["time_bins"],
+        coords={"time_bins": pd.cut(time_bins, time_bins).categories},
+    ).to_dataset(name="val")
+    assert_identical(actual, expected)
+
+
 # TODO: move other groupby tests from test_dataset and test_dataarray over here