Do not prefix an empty messages with tags

If an event's message is completely empty (usually because there was
nothing logged at all), it isn't very useful to output this bare marked
up message at all. We thus ignore it and pass it along unchanged in the
Message encoder.

Note that we still add tags for messages consisting of other whitespace
like e.g. a newline added from a formatted empty message added to a
Logger.

lib/rackstash/encoders/message.rb

@@ -45,8 +45,10 @@ module Rackstash
         normalize_message(event)
         message = event[FIELD_MESSAGE]
 
-        tags = @tagged_fields.map { |key| format_tag event[key] }.compact.join
-        message = message.gsub(/^/) { tags } unless tags.empty?
+        unless message.empty?
+          tags = @tagged_fields.map { |key| format_tag event[key] }.compact.join
+          message = message.gsub(/^/) { tags } unless tags.empty?
+        end
 
         message
       end

spec/rackstash/encoders/message_spec.rb

@@ -43,6 +43,17 @@ describe Rackstash::Encoders::Message do
         event = { 'message' => ["line1\n", "line2\n"] }
         expect(encoder.encode(event)).to eql "line1\nline2\n"
       end
+
+      it 'does not prefix fields on an empty message' do
+        event = { 'message' => [], 'tags' => ['foo', 'bar'] }
+        expect(encoder.encode(event)).to eql ''
+      end
+
+      it 'prefixes fields with a single newline' do
+        event = { 'message' => ["\n"], 'tags' => ['foo', 'bar'] }
+        expect(encoder.encode(event)).to eql "[foo,bar] \n"
+      end
+
     end
   end
 end