As of Minecraft 1.15, shulker heads no longer support transparency.

In previous versions, replacing the texture with transparent pixels would make the shulker head appear transparent (or completely invisible). This no longer works.

Steps to reproduce

1: Create a resource pack with the shulker head texture deleted (resource pack is attached)

2: Summon an invisible shulker with this command: 

/summon minecraft:shulker ~ ~ ~ {active_effects:[{duration:-1,amplifier:0b,id:"minecraft:invisibility"}]}