Beds: allow digging stray top nodes

[SmallJoker]

Stray nodes (or half beds) may occur by ...

1. rotating the bottom bed node without calling the 'on_rotate' callback for various reasons
2. and digging one of the two nodes.
   In master, the top node cannot be dug and is left in a bugged state. You cannot place another bed on top to fix it. The placement aborts. Hence this PR allows digging the top node as well without causing item duplication or item loss.

This PR might or might not fix #3171 (unconfirmed bug)

How to test

Test 1:

1. Check whether normal bed placement and digging works.

Test 2:

1. Rotate the bed (bottom) node using any manipulation tool. I used the technic sonic screwdriver for this.
2. Dig the bottom node. -> You must receive a bed drop
3. Dig the top node. -> There must not be a drop

Test 3:

1. Rotate the bed (bottom) node
2. Dig the top node. -> There must not be a drop
3. Dig the bottom node. -> You must receive a bed drop

Test 4:

1. Rotate the nodes in any way
2. No errors or anomalies must occur.

[SmallJoker]

Fixes #3171 @Niklp09

[sfan5]

tested