Avoid raising errors while running help for custom commands (#944)

* Avoid raising errors while running help for custom commands

Raising an error from the help command is not a pleasure for the
end user, even if the command does not define any attributes

* Update test/irb/command/test_custom_command.rb

---------

Co-authored-by: Stan Lo <stan001212@gmail.com>

lib/irb/command/help.rb

@@ -12,7 +12,7 @@ def execute(command_name)
             help_message
           else
             if command_class = Command.load_command(command_name)
-              command_class.help_message || command_class.description
+              command_class.help_message || command_class.description || ""
             else
               "Can't find command `#{command_name}`. Please check the command name and try again.\n\n"
             end

test/irb/command/test_custom_command.rb

@@ -123,5 +123,31 @@ def execute(*)
       assert_include(output, "2 FooBar executed")
       assert_include(output, "foobar_description")
     end
+
+    def test_no_meta_command_also_works
+      write_ruby <<~RUBY
+        require "irb/command"
+
+        class NoMetaCommand < IRB::Command::Base
+          def execute(*)
+            puts "This command does not override meta attributes"
+            nil
+          end
+        end
+
+        IRB::Command.register(:no_meta, NoMetaCommand)
+
+        binding.irb
+      RUBY
+
+      output = run_ruby_file do
+        type "no_meta\n"
+        type "help no_meta\n"
+        type "exit"
+      end
+
+      assert_include(output, "This command does not override meta attributes")
+      assert_not_include(output, "Maybe IRB bug")
+    end
   end
 end