Add placeholder for code_or_symbol to NullColorizer

The signature of `NullColorizer#wrap` was different from `ConsoleCodes#wrap`, therefore `#message_lines` raised error.
I tried, but it need lots of addition.
Actually, it is almost moving stubs defined here to `FormatterSupport`.

IMO, it is good to make `example` in `FormatterSupport` more full-fledged, but it may affect other tests in future.
Would you give me some advice?

```diff
diff --git a/spec/support/formatter_support.rb b/spec/support/formatter_support.rb
index f0a303c..d2b5dbc 100644
--- a/spec/support/formatter_support.rb
+++ b/spec/support/formatter_support.rb
@@ -189,15 +189,18 @@ module FormatterSupport
   end

   def example
-    result = { :exception => Exception.new }
-    allow(result).to receive(:pending_fixed?) { false }
-    allow(result).to receive(:status) { :passed }
+    result = RSpec::Core::Example::ExecutionResult.new
+    result[:exception]     = begin raise Exception.new("Test exception"); rescue Exception => e; e; end
+    result[:pending_fixed] = false
+    result[:status]        = :passed
     instance_double(RSpec::Core::Example,
                     :description       => "Example",
                     :full_description  => "Example",
                     :execution_result  => result,
                     :location          => "",
-                    :metadata          => {}
+                    :metadata          => {},
+                    :file_path         => __FILE__,
+                    :example_group     => group,
                    )
   end

@@ -206,7 +209,7 @@ module FormatterSupport
   end

   def group
-    class_double "RSpec::Core::ExampleGroup", :description => "Group"
+    class_double "RSpec::Core::ExampleGroup", :description => "Group", :metadata =>{}, :parent_groups => []
   end

   def start_notification(count)
```