Guard Matrix.toString() against a null value

Matrix.ofNull().toString() threw a NullPointerException because
ArrayUtil.getType was called with a null flatArray. Return early with a
flatArray=null rendering instead.

Signed-off-by: Jooyoung Jung <livinglikekrillin@gmail.com>

Author: LivingLikeKrillin

opc-ua-stack/stack-core/src/main/java/org/eclipse/milo/opcua/stack/core/types/builtin/Matrix.java

@@ -250,6 +250,10 @@ public String toString() {
             .add("dataTypeId=" + (dataTypeId != null ? dataTypeId.toParseableString() : null))
             .add("dimensions=" + Arrays.toString(dimensions));
 
+    if (flatArray == null) {
+      return joiner.add("flatArray=null").toString();
+    }
+
     Class<?> clazz = ArrayUtil.getType(flatArray);
 
     if (clazz.isPrimitive()) {

opc-ua-stack/stack-core/src/test/java/org/eclipse/milo/opcua/stack/core/types/builtin/MatrixTest.java

@@ -84,6 +84,15 @@ void matrixToString() {
         boxedMatrix2d.toString());
   }
 
+  @Test
+  void nullMatrixToString() {
+    // A null Matrix has no flatArray; toString() must not throw (previously NPE'd in
+    // ArrayUtil.getType).
+    assertEquals(
+        "Matrix{dataType=null, dataTypeId=null, dimensions=[], flatArray=null}",
+        Matrix.ofNull().toString());
+  }
+
   @Test
   void getDataType() {
     assertEquals(OpcUaDataType.Int32, primitiveMatrix2d.getDataType().orElse(null));