resource(instance): fix provider crash when boot_disk_id is empty

Added a nil pointer check to prevent a nil pointer dereference
causing the provider to crash when `boot_disk_id` is empty and
`disk_attachments` is non-empty.

The following configuration now successfully applies.

```hcl
resource "oxide_instance" "example" {
  project_id       = data.oxide_project.example.id
  description      = "example"
  name             = "example"
  host_name        = "example"
  memory           = 1024 * 1024 * 1024
  ncpus            = 2
  disk_attachments = [oxide_disk.example.id]
}
```

Fixes #466.

Author: sudomateo

internal/provider/resource_instance.go

@@ -1216,6 +1216,10 @@ func newDiskAttachmentsOnCreate(ctx context.Context, client *oxide.Client, diskI
 }
 
 func filterBootDiskFromDisks(disks []oxide.InstanceDiskAttachment, boot_disk *oxide.InstanceDiskAttachment) []oxide.InstanceDiskAttachment {
+	if boot_disk == nil {
+		return disks
+	}
+
 	var filtered_disks = []oxide.InstanceDiskAttachment{}
 	for _, disk := range disks {
 		if disk == *boot_disk {