Cope with Importmap::Packager's return value change

Author: mshibuya

lib/generators/rails_admin/importmap_formatter.rb

@@ -12,6 +12,7 @@ def initialize(path = 'config/importmap.rails_admin.rb')
 
     def format
       imports = packager.import("rails_admin@#{RailsAdmin::Version.js}", from: 'jspm.io')
+      imports = imports[:imports] if imports.key?(:imports)
 
       # Use ESM compatible version to work around https://github.com/cljsjs/packages/issues/1579
       imports['@popperjs/core'].gsub!('lib/index.js', 'dist/esm/popper.js')