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

Reproduce the request using the shown config.ru setup and empty Host header, then trace the request through WEBrick's httpserver.rb and the rackup handler mentioned in the stack trace. Confirm that the malformed request currently reaches Rack with a missing SERVER_NAME and that the completed behavior is an HTTP 400 response instead of 500.