Stop SafeBuffer#clone_empty from issuing warnings

Logic in clone_empty method was dealing with old @dirty variable, which
has changed by @html_safe in this commit:
rails@139963c

This was issuing a "not initialized variable" warning - related to:
rails#5237

The logic applied by this method is already handled by the [] override,
so there is no need to reset the variable here.

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -131,9 +131,7 @@ def initialize_copy(other)
     end
 
     def clone_empty
-      new_safe_buffer = self[0, 0]
-      new_safe_buffer.instance_variable_set(:@dirty, @dirty)
-      new_safe_buffer
+      self[0, 0]
     end
 
     def concat(value)