client/tailscale: add warnings slice to ACLTestFailureSummary

Updates #8645

Signed-off-by: Jenny Zhang <jz@tailscale.com>

client/tailscale/acl.go

@@ -150,8 +150,9 @@ func (c *Client) ACLHuJSON(ctx context.Context) (acl *ACLHuJSON, err error) {
 // ACLTestFailureSummary specifies the JSON format sent to the
 // JavaScript client to be rendered in the HTML.
 type ACLTestFailureSummary struct {
-	User   string   `json:"user"`
+	User     string   `json:"user,omitempty"`
-	Errors []string `json:"errors"`
+	Errors   []string `json:"errors,omitempty"`
+	Warnings []string `json:"warnings,omitempty"`
 }
 
 // ACLTestError is ErrResponse but with an extra field to account for ACLTestFailureSummary.