It's a little bit strange !
If I summon an armor stand with this command: /summon ArmorStand ~ ~ ~ {HandItems:[0:

{id:"minecraft:stone"}

],ShowArms:1} and I do this testfor : /testfor @e[type=ArmorStand] {HandItems:[0:

{id:"minecraft:stone"}

]} it will find the armor stand.

But if I summon an armor stand like that:/summon ArmorStand ~ ~ ~ {HandItems:[0:

{id:"minecraft:air"}

],ShowArms:1} the summon command works but not the testfor ( /testfor @e[type=ArmorStand] {HandItems:[0:

{id:"minecraft:air"}

]} )

I think it's a bug because in the entity data it shows HandItems:[0:

{id:"minecraft:air"}

] .