coretasks, test: handle core.modes setting being None

[dgw]

Description

Tin. Also added None to tested cases, so if it goes back to throwing an error after future changes we'll find out about it.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make lint and make test)
• I have tested the functionality of the things this change touches
  • The corresponding (new) test case test/test_coretasks.py::test_handle_isupport_bot_mode[None] initially failed with an exception traceback, as described in Unspecified modes parameter leads to confusing error #2509.
    With the patch applied, it passes.

Notes

Fun facts about this bug:

• It was introduced in core: better bot-mode handling #2448 (my fault).
• I considered giving core.modes a default='', but it could still become None if someone puts modes = into their config file. The logic changes here would still be required to handle that case, so I decided that None was still a more semantic "not set" value than empty-string and left it without a default value.

[Exirel]

All good! Should have been more careful when reviewing last time!