Do not let effective-pom encoding fall back to null

src/main/java/org/apache/maven/plugins/help/EffectivePomMojo.java

@@ -22,6 +22,7 @@
 
 import java.io.IOException;
 import java.io.StringWriter;
+import java.nio.charset.Charset;
 import java.util.Collections;
 import java.util.List;
 import java.util.Properties;
@@ -107,7 +108,8 @@ public void execute() throws MojoExecutionException {
         }
 
         StringWriter w = new StringWriter();
-        String encoding = output != null ? project.getModel().getModelEncoding() : System.getProperty("file.encoding");
+        String encoding = encodingOrDefault(
+                output != null ? project.getModel().getModelEncoding() : System.getProperty("file.encoding"));
         XMLWriter writer = new PrettyPrintXMLWriter(
                 w, StringUtils.repeat(" ", XmlWriterUtil.DEFAULT_INDENTATION_SIZE), encoding, null);
 
@@ -213,6 +215,13 @@ private static void cleanModel(Model pom) {
         pom.setProperties(properties);
     }
 
+    static String encodingOrDefault(String encoding) {
+        if (encoding == null || encoding.isEmpty()) {
+            return Charset.defaultCharset().name();
+        }
+        return encoding;
+    }
+
     private static class InputLocationStringFormatter extends InputLocation.StringFormatter {
         @Override
         public String toString(InputLocation location) {

src/test/java/org/apache/maven/plugins/help/EffectivePomMojoTest.java

@@ -0,0 +1,41 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.plugins.help;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+class EffectivePomMojoTest {
+
+    @Test
+    void encodingFallbackNeverNull() {
+        String fromNull = EffectivePomMojo.encodingOrDefault(null);
+        assertNotNull(fromNull);
+        assertFalse(fromNull.isEmpty());
+
+        String fromBlank = EffectivePomMojo.encodingOrDefault("");
+        assertNotNull(fromBlank);
+        assertFalse(fromBlank.isEmpty());
+
+        assertEquals("UTF-8", EffectivePomMojo.encodingOrDefault("UTF-8"));
+    }
+}