bug: lacework_alert_rule without subcategories yields an error on apply

[danthegoodman]

Describe the bug
I tried to create a lacework_alert_rule without subcategories. I got an error. I was able to create it online and import it with no changes just fine.

To Reproduce

resource "lacework_alert_rule" "slack" {
  name           = "Slack"
  alert_channels = [local.slack_channel_id]
  severities     = ["Critical", "High"]
  description    = "Send all Crit and High alarms to Slack"
}

Expected behavior
The alert rule to be created correctly.

Screenshots

lacework_alert_rule.slack: Creating...
│ Error: 
│   [POST] https://outschool.lacework.net/api/v2/AlertRules
│   [400] Invalid data: #1: [filters/subcategory: instance type (null) does not match any allowed primitive type (allowed: ["array"])] 
│ 
│   with lacework_alert_rule.slack,
│   on alert-rules.tf line 6, in resource "lacework_alert_rule" "slack":
│    6: resource "lacework_alert_rule" "slack" {
│ 

Please complete the following information):

• OS: registry.gitlab.com/gitlab-org/terraform-images/releases/1.4:v1.0.0
• Provider Version: 1.17.0

Additional context
Add any other context about the problem here.