fix: Make root_block_device.tags default to null

Fixes #440. Without it, if root_block_device is defined without a tags
key, and if enable_volume_tags isn't set to false, a conflict is created
because main.tf:219 defines volume_tags to be at least { Name = ... }
and the root block device gets tags as {}. Making the default null works
around that.

Author: lucasvuotto

variables.tf

@@ -278,7 +278,7 @@ variable "root_block_device" {
     encrypted             = optional(bool)
     iops                  = optional(number)
     kms_key_id            = optional(string)
-    tags                  = optional(map(string), {})
+    tags                  = optional(map(string))
     throughput            = optional(number)
     size                  = optional(number)
     type                  = optional(string)