If entitydata is used on an entity, it ignores empty compound tags directly in HandItems and ArmorItems.
If the entity is in a lazy chunk however it doesn't.

Reproduce by e.g.:
-place a command block with the command 'summon Zombie ~ ~1 ~ {ArmorItems:[{},{},

]}' inside
-place a chain command block with the command 'entitydata @e[r=3] {ArmorItems:[{},{},{},{}]}' inside that the first one points into
-place a redstone block next to the first block
-> result: a zombie with a leather chestplate
-destroy redstone block, tp about 120 blocks away, replace the redstone block using setblock
-tp back or read command feedback
-> result: a zombie without armor