Fallback to StaticAssetFinder

In production environments where assets are precompiled it's possible
for the @assets instance variable to be nil and also Webpacker to not
be used. Falling back to the StaticAssetFinder covers this case.

lib/inline_svg/railtie.rb

@@ -10,15 +10,22 @@ class Railtie < ::Rails::Railtie
 
     config.after_initialize do |app|
       InlineSvg.configure do |config|
-        # In default Rails apps, this will be a fully operational
-        # Sprockets::Environment instance
+        # Configure the asset_finder:
         # Only set this when a user-configured asset finder has not been
         # configured already.
         if config.asset_finder.nil?
           if assets = app.instance_variable_get(:@assets)
+            # In default Rails apps, this will be a fully operational
+            # Sprockets::Environment instance
             config.asset_finder = assets
           elsif defined?(Webpacker)
+            # Use Webpacker when it's available
             config.asset_finder = InlineSvg::WebpackAssetFinder
+          else
+            # Fallback to the StaticAssetFinder if all else fails.
+            # This will be used in cases where assets are precompiled and other
+            # production settings.
+            config.asset_finder = InlineSvg::StaticAssetFinder
           end
         end
       end