2 × 2 trees only check if northwestern-most sapling has a valid block below it

The bug

When a 2 × 2 tree tries to grow it only tests if the block below the northwestern-most (-Z and -X) sapling is valid. This can cause other problems because when a tree grows it replaces all non-dirt blocks with dirt.

Note: This affects in theory oak trees as well because code-wise they can grow as 2 × 2 variant.

How to reproduce

1. Place at -X, -Z and +X, +Z dirt
2. Place at +X, -Z and -X, +Z for example stone
3. Place the saplings on the dirt
4. Use /setblock to place saplings on the stone
5. Use bone meal on the -X, -Z sapling
   → You will see that the tree has grown