Gracefully handle Rack not accepting CLI options (#2630)

In
rack/rack@b95cb72#diff-79892c6315a7f9c02489803e95ce828a8a32759b9f8f196aa98e6826eb562e55R96,
Rack removed the ability to pass options from the command line. It now
fails when you try to do that, and no longer exposes them from
`Builder.load_file`. Puma shouldn't fail when this happens.

This commit falls back to an empty object when Rack does not pass
options back from `Builder.load_file`.

lib/puma/configuration.rb

@@ -343,6 +343,8 @@ def load_rackup
       raise "Missing rackup file '#{rackup}'" unless File.exist?(rackup)
 
       rack_app, rack_options = rack_builder.parse_file(rackup)
+      rack_options = rack_options || {}
+
       @options.file_options.merge!(rack_options)
 
       config_ru_binds = []