Fix: the hisotgrams were adding lines with NaNs when the corresponding partition was completely empty. Now it is back to old behavior and no index is added for partitions completely empty.

src/sp2xr/distribution.py

@@ -259,10 +259,15 @@ def process_hist_and_dist_partition(
     # expand list-of-counts -> wide matrix (one col per bin)
     max_list_length = len(bin_ctrs)
     if df_resampled.empty:
-        # IMPORTANT: use the resampled time index so schema matches later joins
+        """# IMPORTANT: use the resampled time index so schema matches later joins
         idx = flow_dt.index
         columns = [f"result_{i}" for i in range(max_list_length)]
-        ddf_hist = pd.DataFrame(np.nan, columns=columns, index=idx)
+        ddf_hist = pd.DataFrame(np.nan, columns=columns, index=idx)"""
+
+        ddf_hist = pd.DataFrame(
+            columns=[f"result_{i}" for i in range(max_list_length)],
+            index=pd.DatetimeIndex([], name=df.index.name),
+        )
     else:
         ddf_hist = df_resampled["result"].apply(pd.Series)
         ddf_hist.index = df_resampled.index