box/console: handle empty output format

In case if output format is not specified we should exit
with more readable error message.

Fixes #4638

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

src/box/lua/console.lua

@@ -126,6 +126,9 @@ end
 
 local function parse_output(value)
     local fmt, opts
+    if not value then
+        return 'Specify output format: lua or yaml.'
+    end
     if value:match("([^,]+),([^,]+)") ~= nil then
         fmt, opts = value:match("([^,]+),([^,]+)")
     else