box/console: Add mapping for direct symbols

Sole symbols (such as box.NULL) are not processed
by serpent "custom" symbols feature, since they
are not in table.

Thus we should process them separately. Without it
we have

 > require('console').set_default_output("lua,block")
 > ;
 > box.NULL
 > "cdata<void %*>: NULL";

instead of

 > box.NULL
 > box.NULL;

as it should be.

Part-of #3834

Reviewed-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
(cherry picked from commit 6847367f)

src/box/lua/console.lua

@@ -40,12 +40,24 @@ output_handlers["yaml"] = function(status, opts, ...)
     return internal.format({ error = err })
 end
 
+-- A map for internal symbols in case if they
+-- are not inside tables and serpent won't be
+-- able to handle them properly.
+local lua_map_direct_symbols = {
+    [box.NULL]      = 'box.NULL',
+}
+
 output_handlers["lua"] = function(status, opts, ...)
     --
     -- Don't print nil if there is no data
     if not ... then
         return ""
     end
+    for k,v in pairs(lua_map_direct_symbols) do
+        if k == ... then
+            return v
+        end
+    end
     --
     -- Map internal symbols which serpent doesn't know
     -- about to a known representation.