Endermen holding attached stems do not drop seeds

Relates to MC-136360, MC-205885

Endermen holding normal stems drop the equivalent seeds, since the seeds are the item form of the block. However, the same is not true for attached stems - seeds would again be expected here, but nothing is dropped.

To reproduce:

1. /summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:melon_stem}}

2. Kill it and note the dropped seeds

3. /summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:attached_melon_stem}}

4. Kill it and note the dropped nothing