mojo: Re-allow empty callbacks to be passed to the bindings.

mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
index 6d723447cf29ae697162e65bf13fafa6e93d1ab4..32b0a44e7667351ec3d2c7f353a2424b8422d549 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
@@ -145,7 +145,8 @@ bool {{class_name}}_{{method.name}}_ForwardToCallback::Accept(
   {{alloc_params(method.response_param_struct, "params", "message",
                  "&serialization_context_",
                  "{{class_name}}_{{method.name}} response")}}
-  callback_.Run({{pass_params(method.response_parameters)}});
+  if (!callback_.is_null())
+    callback_.Run({{pass_params(method.response_parameters)}});
   return true;
 }
 {%-   endif %}