Advancements correctly require a title if they have a display object (or they won't load). HOWEVER, the title does not have minimum character limit set, allowing the creation of 0-character long titles.

Steps to reproduce:

• Add the following advancement:

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

• Observe how the advancement is valid
• Remove the "title" field
• Observe how the advancement fails to load.

Expected behavior:
Either title is optional, or has a minimum of 1 character required.

Current behavior:
Title is required, but no minimum character limit is present.

Note:
Since advancement titles can use translations, and translations can be empty strings, the better fix would just be to make title optional and make it act as "text":"" when left out.