chore: prevent nil dereferences on cmd handlers (#8319)

* chore: detect nil cmd handlers

Prevent nil panic dereferences on cmd handlers. Add a unit test
to prevent future mistakes

cli/root.go

@@ -260,6 +260,18 @@ func (r *RootCmd) Command(subcommands []*clibase.Cmd) (*clibase.Cmd, error) {
 	// Add a wrapper to every command to enable debugging options.
 	cmd.Walk(func(cmd *clibase.Cmd) {
 		h := cmd.Handler
+		if h == nil {
+			// We should never have a nil handler, but if we do, do not
+			// wrap it. Wrapping it just hides a nil pointer dereference.
+			// If a nil handler exists, this is a developer bug. If no handler
+			// is required for a command such as command grouping (e.g. `users'
+			// and 'groups'), then the handler should be set to the helper
+			// function.
+			//	func(inv *clibase.Invocation) error {
+			//		return inv.Command.HelpHandler(inv)
+			//	}
+			return
+		}
 		cmd.Handler = func(i *clibase.Invocation) error {
 			if !debugOptions {
 				return h(i)