fix(User): only assign to bot initially or if info is actually present

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

Fixes #4985 and fixes #4943.

This PR fixes the bot property of a bot to erroneously changing into false when the USER partial was enabled and their partial is requested through something like Action#getUser or explicitly through UserStore#add with just an id property.

This is solved by only initially setting the property, or updating it if the payload actually includes a bot property

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[advaith1]

Shouldn't this also be done for User#system?