fix: removing last rule from firewall fails with invalid_input error (#…

…696)

If you try to delete last firewall rule, rule is `null` but should be
`[]`, resulting in error message

> hcloud: invalid input in field 'rules' (invalid_input)

`hcloud firewall delete-rule --direction in --protocol tcp --source-ips
'8.8.8.8/32' --port 443 f2b`

internal/cmd/firewall/delete_rule.go

@@ -105,7 +105,7 @@ var DeleteRuleCmd = base.Cmd{
 			}
 		}
 
-		var rules []hcloud.FirewallRule
+		var rules = make([]hcloud.FirewallRule, 0)
 		for _, existingRule := range firewall.Rules {
 			if !reflect.DeepEqual(existingRule, rule) {
 				rules = append(rules, existingRule)

internal/cmd/firewall/delete_rule_test.go

@@ -36,7 +36,7 @@ func TestDeleteRule(t *testing.T) {
 		Get(gomock.Any(), "test").
 		Return(fw, nil, nil)
 	fx.Client.FirewallClient.EXPECT().
-		SetRules(gomock.Any(), fw, hcloud.FirewallSetRulesOpts{Rules: nil}).
+		SetRules(gomock.Any(), fw, hcloud.FirewallSetRulesOpts{Rules: []hcloud.FirewallRule{}}).
 		Return([]*hcloud.Action{{ID: 123}, {ID: 321}}, nil, nil)
 	fx.ActionWaiter.EXPECT().
 		WaitForActions(gomock.Any(), gomock.Any(), []*hcloud.Action{{ID: 123}, {ID: 321}}).