[datadog_security_monitoring_rule] Fix panic when notifications list contains nil values

[shinsj4653]

Description

Fixes a panic that occurs when the notifications list in case or third_party_case blocks contains nil or empty values.

Closes #3322

Changes

• Modified parseStringArray() to use safe type assertion pattern
• Added nil check before type conversion to prevent runtime panic
• Added unit tests to verify the fix

Root Cause

The parseStringArray() function used direct type assertion (value.(string)) which panics when the value is nil. This happens when users specify notifications = [""] or notifications = [null] in their Terraform configuration.

Solution

Applied safe type assertion pattern with nil check:

// Before (panic prone)
parsed[idx] = value.(string)

// After (safe)
if value == nil {
    continue
}
if strVal, ok := value.(string); ok {
    parsed = append(parsed, strVal)
}

Testing

• Unit tests for parseStringArray() - all pass
• Existing acceptance tests - no regression