Avoid error when using in api only mode

[pzgz]

When using in API only mode, running rails g audit_log:install will result in method_missing': undefined method assets' for #<Rails::Application:...`, there might be a better way to detect api only mode application, but for now, simply check the assets attribute existence

[liijunwei]

When using in API only mode, running rails g audit_log:install will result in method_missing': undefined method assets' for #<Rails::Application:...`, there might be a better way to detect api only mode application, but for now, simply check the assets attribute existence

Worked. Thanks