The stepping_on predicate condition does not correctly succeed on trapdoors

23w18a fixed a step sound bug, as well as the stepping_on predicate condition not checking the block the player is "attached" to when shift-crouching far off the side above air/fluid.

However, this is not true for closed bottom-half trapdoors. Even if stood in the centre of the trapdoor, the predicate does not succeed. This is the case regardless of whether a block is underneath the trapdoor.

To reproduce:

1. Create a predicate that only tests for stepping_on trapdoor (attached called step, within the test datapack).
2. Stand on a closed bottom-half trapdoor.
3. Run /execute if predicate test:step.
4. Note that the test fails.