Fix serializing null values (#93)

Author: nhurion

ecs-logging-core/src/main/java/co/elastic/logging/JsonUtils.java

@@ -58,6 +58,10 @@ public final class JsonUtils {
     }
 
     public static void quoteAsString(CharSequence content, StringBuilder sb) {
+        if (content == null) {
+            sb.append("null");
+            return;
+        }
         final int[] escCodes = sOutputEscapes128;
         final int escLen = escCodes.length;
         for (int i = 0, len = content.length(); i < len; ++i) {

ecs-logging-core/src/test/java/co/elastic/logging/EcsJsonSerializerTest.java

@@ -24,6 +24,7 @@
  */
 package co.elastic.logging;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.jupiter.api.Test;
@@ -54,6 +55,13 @@ void serializeExceptionAsString() throws IOException {
         assertThat(jsonNode.get("error.stack_trace").textValue()).isEqualTo(stringWriter.toString());
     }
 
+    @Test
+    void serializeNullDoesNotThrowAnException() throws JsonProcessingException {
+        StringBuilder stringBuilder = new StringBuilder();
+        EcsJsonSerializer.serializeFormattedMessage(stringBuilder, null);
+        assertThat(stringBuilder.toString()).isEqualTo("\"message\":\"null\", ");
+    }
+
     @Test
     void serializeExceptionAsArray() throws IOException {
         Exception exception = new Exception("foo");