voxpupuli#146 leave encrypted value if encryptor returns nil

Adds the possibility to have an encryptor which can not decrypt  everything because the actual user does not have all private keys.

lib/hiera/backend/eyaml/parser/encrypted_tokens.rb

@@ -57,6 +57,12 @@ def to_decrypted(args={})
             label_string = label.nil? ? '' : "#{label}: "
             format = args[:format].nil? ? @format : args[:format]
             index = args[:index].nil? ? '' : "(#{args[:index]})"
+            if @plain_text.nil? then
+              if Utils::hiera? then
+                raise "Encryptor #{@encryptor.tag} could not decrypt #{label_string} and returned nil!"
+              end
+              return to_encrypted(args)
+            end
             case format
               when :block
                 chevron = (args[:use_chevron].nil? || args[:use_chevron]) ? ">\n" : ''