Recurrence rule can be set to null when updating

[Lexedia]

Description

• recurrence_rule can be set to null when updating an event discord/discord-api-docs#7062

Use smart commits here to manipulate issues (eg. Fixes #issue)

Connected issues & other potential problems

If changes in PR are connected to other issues or are affecting code in other parts of framework
(e.g. in main package or any other subpackage) make sure to link issue/PR and describe said problem

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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