fix missing group in exposure

Author: spjuhel

climada/trajectories/riskperiod.py

@@ -710,10 +710,15 @@ def calc_eai_gdf(self) -> gpd.GeoDataFrame:
         df = df.reset_index().melt(
             id_vars="date", var_name="coord_id", value_name="risk"
         )
-        eai_gdf = self.snapshot_start.exposure.gdf[["group_id"]]
-        eai_gdf["coord_id"] = eai_gdf.index
-        eai_gdf = eai_gdf.merge(df, on="coord_id")
-        eai_gdf = eai_gdf.rename(columns={"group_id": "group"})
+        if "group_id" in self.snapshot_start.exposure.gdf:
+            eai_gdf = self.snapshot_start.exposure.gdf[["group_id"]]
+            eai_gdf["coord_id"] = eai_gdf.index
+            eai_gdf = eai_gdf.merge(df, on="coord_id")
+            eai_gdf = eai_gdf.rename(columns={"group_id": "group"})
+        else:
+            eai_gdf = df
+            eai_gdf["group"] = pd.NA
+
         eai_gdf["group"] = pd.Categorical(eai_gdf["group"], categories=self._groups_id)
         eai_gdf["metric"] = "eai"
         eai_gdf["measure"] = self.measure.name if self.measure else "no_measure"