fix(onboarding_role): ensure bot is ready before checking roles in guilds

[tigattack]

Initial Checklist

• Has @tigattack been added as a reviewer?
• If applicable, have the relevant project(s), milestone(s), and label(s) been applied?
• If applicable, have you added details of the cog to the readme as per README.md?

Details

Does this resolve an issue?
Not logged.

Changes

Features / Fixes

• Fixes issue where cog attempts to check for the configured role too early and consequently skips the on-start onboarding.
• Fixes issue where a false-positive warning will fire if no role is configured.