Fix inconsistency in returning list results both in etcd3 and watchcache

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go

@@ -715,6 +715,10 @@ func (c *Cacher) GetList(ctx context.Context, key string, opts storage.ListOptio
 			listVal.Set(reflect.Append(listVal, reflect.ValueOf(elem.Object).Elem()))
 		}
 	}
+	if listVal.IsNil() {
+		// Ensure that we never return a nil Items pointer in the result for consistency.
+		listVal.Set(reflect.MakeSlice(listVal.Type(), 0, 0))
+	}
 	span.AddEvent("Filtered items", attribute.Int("count", listVal.Len()))
 	if c.versioner != nil {
 		if err := c.versioner.UpdateList(listObj, readResourceVersion, "", nil); err != nil {

staging/src/k8s.io/apiserver/pkg/storage/etcd3/store.go

@@ -782,6 +782,10 @@ func (s *store) GetList(ctx context.Context, key string, opts storage.ListOption
 			options = append(options, clientv3.WithRev(withRev))
 		}
 	}
+	if v.IsNil() {
+		// Ensure that we never return a nil Items pointer in the result for consistency.
+		v.Set(reflect.MakeSlice(v.Type(), 0, 0))
+	}
 
 	// instruct the client to begin querying from immediately after the last key we returned
 	// we never return a key that the client wouldn't be allowed to see

staging/src/k8s.io/apiserver/pkg/storage/testing/store_tests.go

@@ -1088,13 +1088,13 @@ func RunTestGetListNonRecursive(ctx context.Context, t *testing.T, store storage
 		name:                 "existing key, resourceVersion=0",
 		key:                  key,
 		pred:                 storage.Everything,
-		expectedAlternatives: [][]example.Pod{nil, {*storedObj}},
+		expectedAlternatives: [][]example.Pod{{}, {*storedObj}},
 		rv:                   "0",
 	}, {
 		name:                 "existing key, resourceVersion=0, resourceVersionMatch=notOlderThan",
 		key:                  key,
 		pred:                 storage.Everything,
-		expectedAlternatives: [][]example.Pod{nil, {*storedObj}},
+		expectedAlternatives: [][]example.Pod{{}, {*storedObj}},
 		rv:                   "0",
 		rvMatch:              metav1.ResourceVersionMatchNotOlderThan,
 	}, {
@@ -1142,7 +1142,7 @@ func RunTestGetListNonRecursive(ctx context.Context, t *testing.T, store storage
 		name:        "non-existing key",
 		key:         "/non-existing",
 		pred:        storage.Everything,
-		expectedOut: nil,
+		expectedOut: []example.Pod{},
 	}, {
 		name: "with matching pod name",
 		key:  "/non-existing",
@@ -1154,7 +1154,7 @@ func RunTestGetListNonRecursive(ctx context.Context, t *testing.T, store storage
 				return nil, fields.Set{"metadata.name": pod.Name}, nil
 			},
 		},
-		expectedOut: nil,
+		expectedOut: []example.Pod{},
 	}}
 
 	for _, tt := range tests {