X rebuild: Don't return nil for empty ΔPPTreeSubSet - that leads to SIGSEGV

--- FAIL: TestΔBTail (0.74s)
    --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R/_→T1/T-T/B0:g-T/B1:e (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x632cc1]

goroutine 238 [running]:
testing.tRunner.func1.2(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000001b00)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1146 +0x4b6
panic(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/runtime/panic.go:965 +0x1b9
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.PPTreeSubSet.ApplyΔ(0xc00040f9b0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/pptreesubset.go:515 +0x41
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔTtail).rebuild(0xc00040f830, 0xc, 0xc00040f500, 0xc000142b40, 0x0, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:371 +0x145
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔBtail).rebuildAll(0xc00015dc00, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:319 +0x18c
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild_TR(0xc000001b00, 0xc000142b40, 0xc00015dc00, 0xc00006a960, 0xc, 0xc0004863c0, 0xc00040f3b0, 0xc00040f5c0, 0xc00040f650, 0xc00040f740, ...)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1127 +0x1b6
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild.func1.1.1(0xc000001b00)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1074 +0xafa
testing.tRunner(0xc000001b00, 0xc000555740)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
        /home/kirr/src/tools/go/go/src/testing/testing.go:1238 +0x2b3
exit status 2

wcfs/internal/xbtree/pptreesubset.go

@@ -482,6 +482,15 @@ type ΔPPTreeSubSet struct {
 	δnchildNonLeafs map[zodb.Oid]int
 }
 
+// NewΔPPTreeSubSet creates new empty ΔPPTreeSubSet.
+func NewΔPPTreeSubSet() *ΔPPTreeSubSet {
+	return &ΔPPTreeSubSet{
+		Del: PPTreeSubSet{},
+		Add: PPTreeSubSet{},
+		δnchildNonLeafs: map[zodb.Oid]int{},
+	}
+}
+
 // Update updates δ to be combination of δ+δ2.
 func (δ *ΔPPTreeSubSet) Update(δ2 *ΔPPTreeSubSet) {
 	δ.Del.UnionInplace(δ2.Del)

wcfs/internal/xbtree/treediff.go

@@ -330,7 +330,7 @@ func treediff(ctx context.Context, root zodb.Oid, δtops SetOid, δZTC SetOid, t
 	defer xerr.Contextf(&err, "treediff %s..%s %s", zconnOld.At(), zconnNew.At(), root)
 
 	δT = map[Key]ΔValue{}
-	δtrack = &ΔPPTreeSubSet{Del: PPTreeSubSet{}, Add: PPTreeSubSet{}, δnchildNonLeafs: map[zodb.Oid]int{}}
+	δtrack = NewΔPPTreeSubSet()
 	δtkeycov = &RangedKeySet{}
 
 	tracefDiff("\ntreediff %s  δtops: %v  δZTC: %v\n", root, δtops, δZTC)
@@ -445,7 +445,7 @@ func diffT(ctx context.Context, A, B *Tree, δZTC SetOid, trackSet PPTreeSubSet)
 	defer xerr.Contextf(&err, "diffT %s %s", xidOf(A), xidOf(B))
 
 	δ = map[Key]ΔValue{}
-	δtrack = &ΔPPTreeSubSet{Del: PPTreeSubSet{}, Add: PPTreeSubSet{}, δnchildNonLeafs: map[zodb.Oid]int{}}
+	δtrack = NewΔPPTreeSubSet()
 	δtkeycov = &RangedKeySet{}
 	defer func() {
 		tracefDiff("  -> δ: %v\n", δ)

wcfs/internal/xbtree/δbtail.go

@@ -451,7 +451,7 @@ func (δTtail *ΔTtail) rebuild1(atPrev zodb.Tid, δZ zodb.ΔRevEntry, trackNew
 
 	// skip opening DB connections if there is no change to this tree
 	if len(δtopsByRoot) == 0 {
-		return nil, nil, nil
+		return NewΔPPTreeSubSet(), &RangedKeySet{}, nil
 	}
 
 	if len(δtopsByRoot) != 1 {