Handle block entities with nil nbt

[Flonja]

Check if the data in a block entity is nil, so that there won't be an assign to nil map panic

Schematic: https://mcbuild.org/schematics/18554:lighthouse
Test code:

file, err := os.Open("test.schematic")
if err != nil {
	panic(err)
}
s, err := schematic.FromReader(file)
if err != nil {
	panic(err)
}

w := srv.World()
<-w.Exec(func(tx *world.Tx) {
	tx.BuildStructure(cube.Pos{}, s)
})

[schphe]

problem still seems to persist with uninitialized block entities getting ticked

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x32f428]

goroutine 78 [running]:
github.com/df-mc/dragonfly/server/block.BrewingStand.Tick({{}, {}, 0x0, 0x0, 0x0, 0x0}, 0x28a12c?, {0x12f, 0x73, 0x135}, ...)
/home/schphe/test/dragonfly/server/block/brewing_stand.go:47 +0x38
github.com/df-mc/dragonfly/server/world.ticker.tickBlocksRandomly({0x40013fbda0?}, 0x404a20db90, {0x4046e05b88, 0x1, 0x0?}, 0x1df)
/home/schphe/test/dragonfly/server/world/tick.go:169 +0x564
github.com/df-mc/dragonfly/server/world.ticker.tick({0x4046423101?}, 0x404a20db90)
/home/schphe/test/dragonfly/server/world/tick.go:82 +0x354
github.com/df-mc/dragonfly/server/world.normalTransaction.Run(...)
/home/schphe/test/dragonfly/server/world/tx.go:243
github.com/df-mc/dragonfly/server/world.(*World).handleTransactions(0x40001a69c0)
/home/schphe/test/dragonfly/server/world/world.go:133 +0x34
created by github.com/df-mc/dragonfly/server/world.Config.New in goroutine 1
/home/schphe/test/dragonfly/server/world/conf.go:110 +0x4f0
exit status 2

[Flonja]

I'd like to get your schematic that you used then

[Flonja]

privately resolved @schphe's issue and is fixed in df-mc/schematic#3 and #1100