out_opensearch_data_stream: Early return on empty body

Due to some reason the body could be empty which results in infinite
jumps between failed buffer flushes. Opensearch returns a 400
with "request body is required".

Signed-off-by: Tim Foerster <tim.foerster@hetzner.com>

Author: tonobo

lib/fluent/plugin/out_opensearch_data_stream.rb

@@ -201,6 +201,8 @@ def write(chunk)
         end
       end
 
+      return if bulk_message.to_s.empty?
+
       params = {
         index: data_stream_name,
         body: bulk_message