Puma will not start up when running against an (as yet) unreleased version of rack

[rorymckinley]

Describe the bug
When running a version of rack built from master, puma will not start. I think this is a result of this earlier change.

Puma config:

threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
threads threads_count, threads_count

port        ENV.fetch("PORT") { 3000 }

environment ENV.fetch("RAILS_ENV") { "development" }

plugin :tmp_restart

To Reproduce

Output from puma -C config/puma.rb hello.ru

Puma starting in single mode...
* Puma version: 5.3.0 (ruby 2.7.3-p183) ("Sweetnighter")
*  Min threads: 5
*  Max threads: 5
*  Environment: development
*          PID: 10867
! Unable to load application: TypeError: no implicit conversion of nil into Hash
Traceback (most recent call last):
	11: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/bin/ruby_executable_hooks:22:in `<main>'
	10: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/bin/ruby_executable_hooks:22:in `eval'
	 9: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/bin/puma:23:in `<main>'
	 8: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/bin/puma:23:in `load'
	 7: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/bin/puma:10:in `<top (required)>'
	 6: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/cli.rb:80:in `run'
	 5: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/launcher.rb:181:in `run'
	 4: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/single.rb:44:in `run'
	 3: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/runner.rb:143:in `load_and_bind'
	 2: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/configuration.rb:267:in `app'
	 1: from /home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/configuration.rb:346:in `load_rackup'
/home/rory/.rvm/gems/ruby-2.7.3@xxxx/gems/puma-5.3.0/lib/puma/configuration.rb:346:in `merge!': no implicit conversion of nil into Hash (TypeError)

Expected behavior
For Puma to run. I realise that I am running it against an unreleased version of rack, but the change that appears to be causing the issue is on master and was made almost a year ago, so it does seem likely that it will be included in the next rack release. I am more than happy to contribute towards solving this, but I am going to need guidance as to what a solution would look like as I know virtually nothing about Puma (or rack).

Desktop (please complete the following information):

• OS: Linux
• Puma Version 5.3.0