Fix `Puma::StateFile#load` returns empty string instead of nil (#2810)

Bug introduced in 2b6968f9d4 #2784

Until 5.5.2, empty values were read as `nil`.
Since 5.6.0, empty values are read as `""`.

lib/puma/state_file.rb

@@ -50,6 +50,7 @@ module Puma
         v = v.strip
         @options[k] =
           case v
+          when ''              then nil
           when /\A\d+\z/       then v.to_i
           when /\A\d+\.\d+\z/  then v.to_f
           else                      v.gsub(/\A"|"\z/, '')

test/test_state_file.rb

@@ -0,0 +1,27 @@
+require_relative "helper"
+require_relative "helpers/tmp_path"
+
+require 'puma/state_file'
+
+class TestStateFile < Minitest::Test
+  include TmpPath
+
+  def test_load_empty_value_as_nil
+    state_path = tmp_path('.state')
+    File.write state_path, <<-STATE
+---
+pid: 123456
+control_url:
+control_auth_token:
+running_from: "/path/to/app"
+    STATE
+
+    sf = Puma::StateFile.new
+    sf.load(state_path)
+    assert_equal 123456, sf.pid
+    assert_equal '/path/to/app', sf.running_from
+    assert_nil sf.control_url
+    assert_nil sf.control_auth_token
+
+  end
+end