avoid crashing in no assets (api only) mode

Rails apps can run in API only mode, which means they do not
have an asset pipeline. If one tries to call config.assets for
an app like that one gets the following crash:

```
NoMethodError: undefined method 'assets' for an instance of Rails::Application::Configuration (NoMethodError)
Did you mean?  asset_host
my-app/config/environment.rb:7:in '<main>'
/bundle:25:in 'Kernel#load'
bundle:25:in '<main>'
```

The code a few lines
above (95-100) ([ref](https://github.com/0robustus1/rails_semantic_logger/blob/master/lib/rails_semantic_logger/engine.rb#L95-L100))
is already written defensively. This does the same for this
invocation of config.assets.

Author: 0robustus1

lib/rails_semantic_logger/engine.rb

@@ -204,7 +204,7 @@ class Engine < ::Rails::Engine
         RailsSemanticLogger::Rack::Logger.started_request_log_level = :info if config.rails_semantic_logger.started
 
         # Silence asset logging by applying a filter to the Rails logger itself, not any of the appenders.
-        if config.rails_semantic_logger.quiet_assets && config.assets.prefix
+        if config.rails_semantic_logger.quiet_assets && config.respond_to?(:assets) && config.assets.prefix
           assets_root                                     = config.relative_url_root.to_s + config.assets.prefix
           assets_regex                                    = %r(\A/{0,2}#{assets_root})
           RailsSemanticLogger::Rack::Logger.logger.filter = ->(log) { log.payload[:path] !~ assets_regex if log.payload }