Tree growth can make logs trigger obervers

If there are some observers face to logs, and tree grow near it, the observer can be triggered.

step to reproduce:

1. build it:

2. bonemeal the sapling

3. notice the observer is triggered

expected behaviour: the tree growth should not trigger the observer, as no block is changed in front of it.

 I have tested jungle, oak, acacia and spruce sapling, and they are affected, too.