Client#messageReactionAdd now relies on the GUILD_MEMBER partial

[monbrey]

Please describe the problem you are having in as much detail as possible:
The changes introduced in #4791 (getUser => getUserFromMember) now requires the GUILD_MEMBER partial in order to emit for uncached Users - in 12.3.1 this was the USER partial.

Include a reproducible code sample here, if possible:

const { Client } = require("discord.js")
const client = new Client({
    partials: ["MESSAGE", "REACTION", "USER"] //failing - add "GUILD_MEMBER" to resolve
});
client.once("ready", () => {
    client.users.cache.clear();
});
client.on("messageReactionAdd", (reaction, user) => {
    console.log(`${user.username} reacted with ${reaction.emoji.name}`);
});
client.login(token);

Further details:

• discord.js version: 12.4.0
• Node.js version: 14.7.0
• Operating system: Windows 10
• Priority this issue should have – please be realistic and elaborate if possible: medium, workaround available

Relevant client options:

• partials: ["MESSAGE", "REACTION", "USER"]
• gateway intents: none
• other: none

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