JSON layout shouldn't fail to render nullable strings (#232)

loki-client/src/main/java/com/github/loki4j/pkg/dslplatform/json/RawJsonWriter.java

@@ -321,6 +321,21 @@ public final void writeAsciiString(final String value) {
         position += len;
     }
 
+    /**
+     * Optimized method for writing 'null' into the JSON.
+     */
+    public final void writeNull() {
+        if ((position + 4) >= buffer.length) {
+            enlargeOrFlush(position, 0);
+        }
+        final int s = position;
+        final byte[] _result = buffer;
+        _result[s] = 'n';
+        _result[s + 1] = 'u';
+        _result[s + 2] = 'l';
+        _result[s + 3] = 'l';
+        position += 4;
+    }
 
     /**
      * Content of buffer can be copied to another array of appropriate size.

loki-logback-appender/src/main/java/com/github/loki4j/logback/json/JsonEventWriter.java

@@ -31,10 +31,13 @@ public void writeFieldSeparator() {
     public void writeStringField(String fieldName, String value) {
         raw.writeAsciiString(fieldName);
         raw.writeByte(SEMI);
-        raw.writeString(value);
+        if (value != null)
+            raw.writeString(value);
+        else
+            raw.writeNull();
     }
 
-    public void writeNumberField(String fieldName, Long value) {
+    public void writeNumberField(String fieldName, long value) {
         raw.writeAsciiString(fieldName);
         raw.writeByte(SEMI);
         NumberConverter.serialize(value, raw);

loki-logback-appender/src/test/java/com/github/loki4j/logback/json/JsonEventWriterTest.java

@@ -0,0 +1,30 @@
+package com.github.loki4j.logback.json;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class JsonEventWriterTest {
+
+    @Test
+    public void testWriteMultipleFields() {
+        var writer = new JsonEventWriter(10);
+        writer.writeBeginObject();
+        writer.writeStringField("str", "abc");
+        writer.writeFieldSeparator();
+        writer.writeNumberField("num", 123);
+        writer.writeEndObject();
+
+        assertEquals("{\"str\":\"abc\",\"num\":123}", writer.toString());
+    }
+
+    @Test
+    public void testWriteNullString() {
+        var writer = new JsonEventWriter(10);
+        writer.writeBeginObject();
+        writer.writeStringField("str", null);
+        writer.writeEndObject();
+
+        assertEquals("{\"str\":null}", writer.toString());
+    }
+}