fix: check for `command_prefix=None` before warning about message content intent

[shiftinv]

Summary

Shows a deprecation warning instead of message content intent warning when using Bot(command_prefix=None), and improves the docs regarding None prefixes.

Resolves #688.

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running task lint
  • [x] I have type-checked the code by running task pyright
• [x] This PR fixes an issue
• [ ] This PR adds something new (e.g. new method or parameters)
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[onerandomusername]

I don't think we should fully remove the warning. Instead of displaying the message content intent warning, we should instead use a warning to use InteractionBot and None as the prefix is deprecated.

[shiftinv]

I don't think we should fully remove the warning. Instead of displaying the message content intent warning, we should instead use a warning to use InteractionBot and None as the prefix is deprecated.

Fair enough, changed it to a deprecation warning instead.