While the "icon" field in an advancement can be set to air, showing no/an empty icon in the menu, it cannot be left out to have the same effect.

Steps to reproduce:

• Create the advancement

  {
      "display": {
          "icon": {
              "item": "minecraft:air"
          },
          "title": {
              "text": "A title"
          },
          "description": {
              "text": "A description"
          },
          "background": "minecraft:textures/gui/advancements/backgrounds/stone.png"
      },
      "criteria": {
          "tick": {
              "trigger": "minecraft:tick"
          }
      }
  }

• Observe the advancement menu shows an empty icon
• Remove the "item" field of the icon
• Observe how the advancement fails to load
• Remove the "icon" field altogether
• Observe how the advancement still fails to load

Expected behavior:
If no icon field is provided (or no item provided), it'd show no/an empty icon, just as if the icon is set to air.

Actual result:
The icon field is required and required an icon type field (item).