Internal Server Error 500 when Empty Host Header Sent

Overview

When sent a GET request with an empty host header, WEBrick responds with error code 500, but it should respond with Error Code 400, for an invalid request.

Reproducing the Bug

1. Clone, build, and install WEBrick. (Tested on ea124002ee5ff43b6cf846931b2cb898b3f47819)
2. Set up a config.ru file with the following contents:

class Server
  def call(env)
    [200, {}, []]
  end
end

run Server.new

3. Run rackup.
4. Send a GET request with an empty host header, and observe WEBrick's response as follows:

printf 'GET / HTTP/1.1\r\nHost: \r\n\r\n' | ncat localhost 9292

Rack::Lint::LintError: env missing required key SERVER_NAME (Rack::Lint::LintError)
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:283:in `block in check_environment'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:282:in `each'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:282:in `check_environment'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:63:in `response'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:41:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/show_exceptions.rb:31:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/common_logger.rb:43:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/content_length.rb:20:in `call'
	/usr/share/rubygems-integration/all/gems/rackup-2.1.0/lib/rackup/handler/webrick.rb:111:in `service'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/httpserver.rb:140:in `service'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/httpserver.rb:96:in `run'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/server.rb:310:in `block in start_thread'
::1 - - [27/Aug/2025:18:40:52 +0000] "GET / HTTP/1.1" 500 1183 0.0002

Riproduci la richiesta utilizzando la configurazione config.ru mostrata e un header Host vuoto, quindi segui la richiesta attraverso httpserver.rb di WEBrick e il gestore rackup menzionato nello stack trace. Conferma che la richiesta malformata attualmente raggiunge Rack con un SERVER_NAME mancante e che il comportamento finale è una risposta HTTP 400 invece di 500.