writer: preserve empty events

CloudWatch logs requires that events have a length > 0, so an empty
event is represented as a string containing a single null byte.

Previously, empty events were ignored. However, my use case cares about
formatting, so I've decided to "empty" events (i.e., lines that just
contain a newline character)

writer/writer.go

@@ -280,7 +280,7 @@ func (w *LogWriter) readLines() {
 
 func (w *LogWriter) appendEvent(text string) {
 	if text == "" {
-		return
+		text = "\u0000"
 	}
 
 	w.Lock()

writer/writer_test.go

@@ -123,6 +123,33 @@ func TestWriter(t *testing.T) {
 				},
 			},
 		},
+		{
+			"empty lines",
+			newTestInput([][]byte{
+				[]byte("test input\n"),
+				[]byte("\n"),
+				[]byte("more input\n"),
+				[]byte("\n"),
+			}),
+			Events{
+				{
+					Message:   aws.String("test input"),
+					Timestamp: aws.Int64(1),
+				},
+				{
+					Message:   aws.String("\u0000"),
+					Timestamp: aws.Int64(2),
+				},
+				{
+					Message:   aws.String("more input"),
+					Timestamp: aws.Int64(3),
+				},
+				{
+					Message:   aws.String("\u0000"),
+					Timestamp: aws.Int64(4),
+				},
+			},
+		},
 	}
 
 	for _, c := range cases {