fix: required activities field is not being sent in presence update payload

[priyanuj-gogoi]

Description

Fixes an issue where the bot fails to connect the gateway and logs Unresumable invalid session when PresenceBuilder() activities is ignored.

PresenceBuilder(
  since: DateTime.now(),
  // activities: []
  status: CustomUserStatus.online,
  isAfk: false
);

When sending a presence update event payload, the activities field is supposed to send. Check the docs, there is no ? after activities.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works