Guard Mapper.write against null writer

A `null` writer passed to the `Mapper.write` method, sensibly, throws
an NPE upon flushing the buffer out. However, since the flushing
occurs at a later time, debugging that this occurred is challenging.
Thus, this commit implements a fail-fast `null` guard.

johnzon-mapper/src/main/java/org/apache/johnzon/mapper/Mapper.java

@@ -52,6 +52,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Objects;
 
 import static org.apache.johnzon.mapper.internal.Streams.noClose;
 
@@ -166,7 +167,7 @@ public JsonValue toStructure(final Object object) {
 
     public void writeObject(final Object object, final Writer stream) {
         if (object == null) {
-            try (final Writer w = stream) {
+            try (final Writer w = Objects.requireNonNull(stream, "stream")) {
                 w.write("null");
             } catch (final IOException e) {
                 throw new MapperException(e);
@@ -175,7 +176,7 @@ public void writeObject(final Object object, final Writer stream) {
         }
         final Adapter adapter = config.findAdapter(object.getClass());
         if (adapter != null && TypeAwareAdapter.class.isInstance(adapter) && TypeAwareAdapter.class.cast(adapter).getTo() == JsonString.class) {
-            writeObject(adapter.from(object), stream);
+            writeObject(adapter.from(object), Objects.requireNonNull(stream, "stream"));
             return;
         }
         try (final JsonGenerator generator = generatorFactory.createGenerator(stream(stream))) {

johnzon-mapper/src/test/java/org/apache/johnzon/mapper/MapperTest.java

@@ -30,6 +30,7 @@
 import java.io.ByteArrayOutputStream;
 import java.io.StringReader;
 import java.io.StringWriter;
+import java.io.Writer;
 import java.lang.reflect.ParameterizedType;
 import java.math.BigDecimal;
 import java.util.ArrayList;
@@ -52,6 +53,7 @@
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 import jakarta.json.Json;
@@ -1381,4 +1383,17 @@ public static class Parent {
     public static class Child extends Parent {
         public List<String> children;
     }
+
+    /**
+     * Tests Mapper.writeObject with null Writer.
+     */
+    @Test
+    public void testNullWriter () {
+        assertThrows(NullPointerException.class, () -> {
+            new MapperBuilder().build().writeObject(null, (Writer) null);
+        });
+        assertThrows(NullPointerException.class, () -> {
+            new MapperBuilder().build().writeObject("", (Writer) null);
+        });
+    }
 }