stand/lua: Don't descend into an empty kernels submenu

This submenu is likely going to go away in favor of kernel selection as it
is done in forth at the moment, but for the time being don't descend into it
if we have no kernels available for listing.

stand/lua/menu.lua

@@ -218,6 +218,9 @@ menu.welcome = {
 
 			-- dynamically build the kernel menu:
 			local kernels = core.kernelList();
+			if #kernels == 0 then
+				return nil;
+			end
 			for k, v in ipairs(kernels) do
 				menu.kernel_options[#menu.kernel_options + 1] = {
 					entry_type = "entry",