Steps to Reproduce:
1. Summon an armor stand holding any item:

/summon armor_stand ~ ~ ~ {ShowArms:1b,HandItems:[{id:"minecraft:diamond",count:1b}]}

2. Attempt to change its count value to 0:

/data modify entity @n[type=armor_stand] HandItems[0].count set value 0

Observed Results:
The item count stays as 1

Expected Results:
The item should disappear

Notes:
Setting the item count higher first, and then attempting to set it to 0, shows that it results in 1 instead of doing nothing at all