Support ruby3.3 Logger by properly initialize super class

Upcoming ruby3.3 will have enhanced Logger class:
ruby/ruby@194520f
which initializes newly added instance variables.
Without initializing such variables (in super class), now using
subclass of Logger will cause exception.

To avoid this, first call super, then execute additional subclass
initialization.

Closes asciidoctor#4493 .

lib/asciidoctor/logging.rb

@@ -42,6 +42,7 @@ class MemoryLogger < ::Logger
   attr_reader :messages
 
   def initialize
+    super nil
     self.level = UNKNOWN
     @messages = []
   end
@@ -69,6 +70,7 @@ class NullLogger < ::Logger
   attr_reader :max_severity
 
   def initialize
+    super nil
     self.level = UNKNOWN
   end