Fix key error on empty summary observations in plotter

Co-authored-by: Eivind Jahren <[email protected]>

src/ert/dark_storage/common.py

@@ -294,7 +294,7 @@ def get_observation_keys_for_response(
 
     elif (
         displayed_response_key
-        in ensemble.experiment.response_type_to_response_keys["summary"]
+        in ensemble.experiment.response_type_to_response_keys.get("summary", {})
     ):
         response_key = displayed_key_to_response_key["summary"](displayed_response_key)[
             0

tests/ert/unit_tests/gui/tools/plot/test_plot_api.py

@@ -326,3 +326,14 @@ def test_that_multiple_observations_are_parsed_correctly(api):
     ensemble = next(x for x in api.get_all_ensembles() if x.id == "ens_id_5")
     obs_data = api.observations_for_key([ensemble.id], "WOPR:OP1")
     assert obs_data.shape == (3, 6)
+
+
+def test_that_observations_for_empty_ensemble_returns_empty_data(api_and_storage):
+    api, storage = api_and_storage
+    experiment = storage.create_experiment(
+        parameters=[],
+        responses=[SummaryConfig(name="summary", input_files=[""], keys=["NAIMFRAC"])],
+        observations={},
+    )
+    ensemble = storage.create_ensemble(experiment.id, ensemble_size=1)
+    assert api.observations_for_key([str(ensemble.id)], "NAIMFRAC").empty