Ensure NID slices are never nil

Otherwise we have to get the storage functions to convert this to a
nonnil slice before we insert in the DB.

state/accumulator_test.go

@@ -463,7 +463,8 @@ func TestCalculateNewSnapshotDupe(t *testing.T) {
 				Type:     "a",
 				StateKey: "b",
 			},
-			wantOtherNIDs: []int64{2},
+			wantOtherNIDs:  []int64{2},
+			wantMemberNIDs: []int64{},
 		},
 		// basic addition
 		{
@@ -479,7 +480,8 @@ func TestCalculateNewSnapshotDupe(t *testing.T) {
 				Type:     "c",
 				StateKey: "d",
 			},
-			wantOtherNIDs: []int64{1, 2},
+			wantOtherNIDs:  []int64{1, 2},
+			wantMemberNIDs: []int64{},
 		},
 		// dupe nid
 		{

state/event_table.go

@@ -84,7 +84,12 @@ func (ev *Event) ensureFieldsSetOnEvent() error {
 
 type StrippedEvents []Event
 
+// NIDs partitions the nids in this set of stripped events into the nids of membership
+// events and all others. The returned lists inherit their order from the given slice of
+// StrippedEvents, and are never nil.
 func (se StrippedEvents) NIDs() (membershipNIDs, otherNIDs []int64) {
+	membershipNIDs = make([]int64, 0, len(se))
+	otherNIDs = make([]int64, 0, len(se))
 	for _, s := range se {
 		if s.Type == "m.room.member" {
 			membershipNIDs = append(membershipNIDs, s.NID)