Add default empty list for allowed and not_allowed in var.maintenance_window

[lonegunmanb]

Describe your changes

Issue number

#455

Checklist before requesting a review

• The pr title can be used to describe what this pr did in CHANGELOG.md file
• I have executed pre-commit on my machine
• I have passed pr-check on my machine

Thanks for your cooperation!

[github-actions]

Potential Breaking Changes in 547fd7a:
[update] "Variables.maintenance_window.Type" from 'object({
allowed = list(object({
day = string
hours = set(number)
})),
not_allowed = list(object({
end = string
start = string
})),
})' to 'object({
allowed = optional(list(object({
day = string
hours = set(number)
})), []),
not_allowed = optional(list(object({
end = string
start = string
})), []),
})'

[jiaweitao001]

LGTM.