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

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- import "struct_macros.tmpl" as struct_macros %}
 
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 {%- set namespace_as_string = "%s"|format(namespace|replace(".","::")) %}
 
 {%- macro alloc_params(struct, params, message, serialization_context,
                        description) %}
   ({{params}})->DecodePointers();
@@ skipping 127 matching lines @@
 };
 bool {{class_name}}_{{method.name}}_ForwardToCallback::Accept(
     mojo::Message* message) {
   internal::{{class_name}}_{{method.name}}_ResponseParams_Data* params =
       reinterpret_cast<internal::{{class_name}}_{{method.name}}_ResponseParams_Data*>(
           message->mutable_payload());
 
   {{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 %}
 {%- endfor %}
 
 {{proxy_name}}::{{proxy_name}}(mojo::MessageReceiverWithResponder* receiver)
     : ControlMessageProxy(receiver) {
 }
 
 {#--- Proxy definitions #}
@@ skipping 310 matching lines @@
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       &validation_context,
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}