[JENKINS-50616] - Add org.jruby.RubyNil to the whitelist (#3404)

* [JENKINS-50616] - Add org.jruby.RubyNil to the whitelist

* [JENKINS-50616] - Fix the typo in the comment

core/src/main/resources/jenkins/security/whitelisted-classes.txt

@@ -175,7 +175,13 @@ org.jenkinsci.jruby.JRubyMapper$DynamicProxy
 org.jenkinsci.lib.xtrigger.XTriggerCause
 org.jenkinsci.lib.xtrigger.XTriggerCauseAction
 
-# TODO remove (also XStream2.BlacklistedTypesConverter.JRUBY_PROXY) when https://github.com/jenkinsci/ruby-runtime-plugin/pull/5 is widely adopted
+# TODO remove (also XStream2.BlacklistedTypesConverter.JRUBY_PROXY) when Ruby Runtime is fixed
+# Related PRs:
+#   - https://github.com/jenkinsci/ruby-runtime-plugin/pull/5,
+#   - https://github.com/jenkinsci/ruby-runtime-plugin/pull/6
+#
+# oleg-nenashev in PR#6 we are trying to get help from last maintainers due to the plugin codebase splitbrain.
+#               It is required to fix JENKINS-50616 in a proper way for 2.107.x
 org.jruby.RubyArray
 org.jruby.RubyBignum
 org.jruby.RubyBoolean
@@ -183,6 +189,7 @@ org.jruby.RubyBoolean$False
 org.jruby.RubyBoolean$True
 org.jruby.RubyFixnum
 org.jruby.RubyHash
+org.jruby.RubyNil
 org.jruby.RubyObject
 org.jruby.RubyString
 org.jruby.RubySymbol