Xpp3Dom.getValue(): honor @NonNull contract

src/main/java/org/apache/maven/shared/utils/xml/Xpp3Dom.java

@@ -95,6 +95,10 @@ public Xpp3Dom(String name) {
         childMap = new HashMap<>();
     }
 
+    boolean isValueSet() {
+        return value != null;
+    }
+
     /**
      * Create instance.
      *
@@ -140,7 +144,7 @@ public String getName() {
      */
     @NonNull
     public String getValue() {
-        return value;
+        return value != null ? value : "";
     }
 
     /**

src/main/java/org/apache/maven/shared/utils/xml/Xpp3DomWriter.java

@@ -72,8 +72,8 @@ public static void write(XMLWriter xmlWriter, Xpp3Dom dom, boolean escape) throw
             write(xmlWriter, aChildren, escape);
         }
 
-        String value = dom.getValue();
-        if (value != null) {
+        if (dom.isValueSet()) {
+            String value = dom.getValue();
             if (escape) {
                 xmlWriter.writeText(value);
             } else {

src/test/java/org/apache/maven/shared/utils/xml/pull/Xpp3DomTest.java

@@ -27,7 +27,7 @@
 import static org.apache.maven.shared.utils.xml.Xpp3Dom.mergeXpp3Dom;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
-import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 /**
@@ -42,6 +42,13 @@ private Xpp3Dom createElement(String element, String value) {
         return t1s1;
     }
 
+    @Test
+    public void defaultValueIsNotNull() {
+        // getValue() is annotated @NonNull but the one-arg constructor
+        // did not initialize value, violating the contract.
+        assertNotNull(new Xpp3Dom("test").getValue());
+    }
+
     @Test
     public void mergePrecedenceSelfClosed() throws XmlPullParserException {
         Xpp3Dom parentConfig = build("<configuration><items><item/></items></configuration>");
@@ -82,7 +89,7 @@ public void selfOverrideOnRootNode() {
         Xpp3Dom result = mergeXpp3Dom(t1, t2);
 
         assertEquals(2, result.getAttributeNames().length);
-        assertNull(result.getValue());
+        assertEquals("", result.getValue());
     }
 
     @Test