logger/gelf: Skip empty lines to comply with spec

The [gelf payload specification](http://docs.graylog.org/en/2.4/pages/gelf.html#gelf-payload-specification)
demands that the field `short_message` *MUST* be set by the client library.
Since docker logging via the gelf driver sends messages line by line, it can happen that messages with an empty
`short_message` are passed on. This causes strict downstream processors (like graylog) to raise an exception.

The logger now skips messages with an empty line.

Resolves: #40232
See also: #37572

Signed-off-by: Jonas Heinrich <Jonas@JonasHeinrich.com>

daemon/logger/gelf/gelf.go

@@ -166,6 +166,10 @@ func newGELFUDPWriter(address string, info logger.Info) (gelf.Writer, error) {
 }
 
 func (s *gelfLogger) Log(msg *logger.Message) error {
+	if len(msg.Line) == 0 {
+		return nil
+	}
+
 	level := gelf.LOG_INFO
 	if msg.Source == "stderr" {
 		level = gelf.LOG_ERR