Support non-webpacker environments (#582)

Webpacker can be defined, but not configured.

In this case, treating it like webpacker is not present

```
lib/mini_profiler_rails/railtie_methods.rb:55:in `get_webpacker_assets_path': undefined method `config' for Webpacker:Module (NoMethodError)
Did you mean?  concerning
	from lib/mini_profiler_rails/railtie.rb:33:in `initialize!'
```

solution:

ensure the full chain is available

lib/mini_profiler_rails/railtie_methods.rb

@@ -52,7 +52,7 @@ def should_move?(child, node)
   end
 
   def get_webpacker_assets_path
-    if defined?(Webpacker) && Webpacker.config.config_path.exist?
+    if defined?(Webpacker) && Webpacker.try(:config)&.config_path&.exist?
       Webpacker.config.public_output_path.to_s.gsub(Webpacker.config.public_path.to_s, "")
     end
   end