fix(lua): correct return value for on_key with no arguments

(cherry picked from commit 3edc8417f0)

test/functional/lua/vim_spec.lua

@@ -2339,6 +2339,12 @@ describe('lua stdlib', function()
     end)
 
     it('allows removing on_key listeners', function()
+      -- Create some unused namespaces
+      meths.create_namespace('unused1')
+      meths.create_namespace('unused2')
+      meths.create_namespace('unused3')
+      meths.create_namespace('unused4')
+
       insert([[hello world]])
 
       exec_lua [[