events: Give non-null tipsets in handlers

chain/events/events_height.go

@@ -77,7 +77,7 @@ func (e *heightEvents) headChangeAt(rev, app []*types.TipSet) error {
 				hnd := e.heightTriggers[tid]
 				triggerH := h - uint64(hnd.confidence)
 
-				incTs, err := e.tsc.get(triggerH)
+				incTs, err := e.tsc.getNonNull(triggerH)
 				if err != nil {
 					return err
 				}

chain/events/events_test.go

@@ -241,7 +241,7 @@ func TestAtNullTrigger(t *testing.T) {
 	var reverted bool
 
 	err := events.ChainAt(func(ts *types.TipSet, curH uint64) error {
-		require.Nil(t, ts)
+		require.Equal(t, uint64(6), ts.Height())
 		require.Equal(t, 8, int(curH))
 		applied = true
 		return nil

chain/events/tscache.go

@@ -76,6 +76,19 @@ func (tsc *tipSetCache) revert(ts *types.TipSet) error {
 	return nil
 }
 
+func (tsc *tipSetCache) getNonNull(height uint64) (*types.TipSet, error) {
+	for {
+		ts, err := tsc.get(height)
+		if err != nil {
+			return nil, err
+		}
+		if ts != nil {
+			return ts, nil
+		}
+		height++
+	}
+}
+
 func (tsc *tipSetCache) get(height uint64) (*types.TipSet, error) {
 	if tsc.len == 0 {
 		return nil, xerrors.New("tipSetCache.get: cache is empty")