Mob spawners no longer spawn mobs in air blocks

This bug only came about in 19w13a, mob spawners worked properly in 19w12b.

In order to reproduce,

1. Find a spawner (blaze, zombie etc.)
2. Clear around the spawner so there are no blocks, at least 9x9 area.
   → You will see spawns only happen on the spawner, if there is no block blocking a spawn from occurring.