Fix interface of baseclass and null object

lib/mutant/reporter.rb

@@ -3,6 +3,16 @@ module Mutant
   class Reporter
     include Adamantium::Flat, AbstractType
 
+    # Write warning message
+    #
+    # @param [String] message
+    #
+    # @return [self]
+    #
+    # @api private
+    #
+    abstract_method :warn
+
     # Report object
     #
     # @param [Object] object

lib/mutant/reporter/null.rb

@@ -5,6 +5,18 @@ module Mutant
     class Null < self
       include Equalizer.new
 
+      # Write warning message
+      #
+      # @param [String] _message
+      #
+      # @return [self]
+      #
+      # @api private
+      #
+      def report(_message)
+        self
+      end
+
       # Report object
       #
       # @param [Object] _object