in_tail: Fix check condition for encoding

Signed-off-by: Masahiro Nakagawa <[email protected]>

lib/fluent/plugin/in_tail.rb

@@ -171,7 +171,7 @@ def configure_encoding
 
       @encoding = parse_encoding_param(@encoding) if @encoding
       @from_encoding = parse_encoding_param(@from_encoding) if @from_encoding
-      if @encoding == @from_encoding
+      if @encoding && (@encoding == @from_encoding)
         log.warn "'encoding' and 'from_encoding' are same encoding. No effect"
       end
     end