Fix all-nan slices in ExtremeValues (#1983)

### What kind of change does this PR introduce?

* All-nan slices were previously broken in `_extremes_train_1d`, now
there is a fast-track that treats all-nan separately.

### Does this PR introduce a breaking change?
No

CHANGELOG.rst

@@ -16,6 +16,7 @@ Bug fixes
 * Fixed a units formatting bug with indicators that output "delta" Celsius degrees. (:pull:`1973`).
 * Corrected the ``"choices"`` of parameter ``op`` in the docstring of ``frost_free_spell_max_length``. (:pull:`1977`).
 * Reorganised how ``Indicator`` subclasses can added arguments to the call signature. Injecting such arguments now works. For xclim's subclasses, this bug only affected the ``indexer`` argument of indicators subclassing ``xc.core.indicator.IndexingIndicator``. (:pull:`1981`).
+* All-nan slices are now treated correctly in method `ExtremeValues`. (:issue:`1982`, :pull:`1983`).
 
 v0.53.1 (2024-10-21)
 --------------------

tests/test_sdba/test_adjustment.py

@@ -725,6 +725,8 @@ def dist(ref, sim):
         assert (ref - scen).mean().tasmin < 5e-3
 
 
+# TODO: below we use `.adjust(scen,sim)`, but in the function signature, `sim` comes before
+# are we testing the right thing below?
 class TestExtremeValues:
     @pytest.mark.parametrize(
         "c_thresh,q_thresh,frac,power",
@@ -805,6 +807,19 @@ def test_real_data(self, open_dataset):
         new_scen = EX.adjust(scen, hist, frac=0.000000001)
         new_scen.load()
 
+    def test_nan_values(self):
+        times = xr.cftime_range("1990-01-01", periods=365, calendar="noleap")
+        ref = xr.DataArray(
+            np.arange(365),
+            dims=("time"),
+            coords={"time": times},
+            attrs={"units": "mm/day"},
+        )
+        hist = (ref.copy() * np.nan).assign_attrs(ref.attrs)
+        EX = ExtremeValues.train(ref, hist, cluster_thresh="10 mm/day", q_thresh=0.9)
+        new_scen = EX.adjust(sim=hist, scen=ref)
+        assert new_scen.isnull().all()
+
 
 class TestOTC:
     def test_compare_sbck(self, random, series):

xclim/sdba/_adjustment.py

@@ -778,10 +778,14 @@ def _fit_on_cluster(data, thresh, dist, cluster_thresh):
 
 def _extremes_train_1d(ref, hist, ref_params, *, q_thresh, cluster_thresh, dist, N):
     """Train for method ExtremeValues, only for 1D input along time."""
+    # Fast-track, do nothing for all-nan slices
+    if all(np.isnan(ref)) or all(np.isnan(hist)):
+        return np.full(N, np.nan), np.full(N, np.nan), np.nan
+
     # Find quantile q_thresh
     thresh = (
-        np.quantile(ref[ref >= cluster_thresh], q_thresh)
-        + np.quantile(hist[hist >= cluster_thresh], q_thresh)
+        np.nanquantile(ref[ref >= cluster_thresh], q_thresh)
+        + np.nanquantile(hist[hist >= cluster_thresh], q_thresh)
     ) / 2
 
     # Fit genpareto on cluster maximums on ref (if needed) and hist.