CategoryChannel#parentID is sometimes undefined

[Jiralite]

Please describe the problem you are having in as much detail as possible:
The .parentID property of a category is sometimes undefined, but it should always be null.

I tested in all guilds my bot was in and I was surprised to see a mixture of undefined and null values.

Include a reproducible code sample here, if possible:

const client = new (require("discord.js").Client)();

client.on("ready", () => {
  const results = [];

  client.guilds.cache.forEach(({ channels: { cache } }) => {
    results.push(cache.filter(({ type }) => type === "category").map(({ parentID }) => parentID));
  });

  console.log(results);
});

client.login("token");

results resulted as:

[
  [ undefined, undefined, undefined ],
  [ undefined, undefined ],
  [
    null, null, null,
    null, null, null,
    null, null, null
  ],
  [
    null,      undefined,
    undefined, null,
    undefined, null,
    undefined, null
  ],
  [ undefined, undefined, null ],
  [ undefined, undefined ],
  [ null, null, null, null, null ],
  [ null, null ],
  [ undefined, undefined ]
]

Further details:

• discord.js version: 12.3.1
• Node.js version: 14.10.0
• Operating system: macOS 10.15.6
• Priority this issue should have – please be realistic and elaborate if possible: Low

Relevant client options: N/A

• partials: none
• gateway intents: none
• other: none

• I have also tested the issue on latest master, commit hash: d234165

[almostSouji]

seems to be yet another case of #4593 | #4706
where discord simply doesn't send fields at all instead of as null values

CR in 12.3.1 with client.channels.cache.filter(c => c.type !== "category" && c.parentID === undefined).size