[refactor] Make it clear that adding to an EmptyNodeSet is not possible

Author: adamretter

exist-core/pom.xml

@@ -889,6 +889,7 @@
                                 <include>src/main/java/org/exist/dom/persistent/DocumentSet.java</include>
                                 <include>src/main/java/org/exist/dom/persistent/DocumentTypeImpl.java</include>
                                 <include>src/main/java/org/exist/dom/persistent/ElementImpl.java</include>
+                                <include>src/main/java/org/exist/dom/persistent/EmptyNodeSet.java</include>
                                 <include>src/main/java/org/exist/dom/persistent/LockToken.java</include>
                                 <include>src/main/java/org/exist/dom/persistent/Match.java</include>
                                 <include>src/main/java/org/exist/dom/persistent/NewArrayNodeSet.java</include>
@@ -1589,6 +1590,7 @@
                                 <exclude>src/main/java/org/exist/dom/persistent/DocumentSet.java</exclude>
                                 <exclude>src/main/java/org/exist/dom/persistent/DocumentTypeImpl.java</exclude>
                                 <exclude>src/main/java/org/exist/dom/persistent/ElementImpl.java</exclude>
+                                <exclude>src/main/java/org/exist/dom/persistent/EmptyNodeSet.java</exclude>
                                 <exclude>src/main/java/org/exist/dom/persistent/LockToken.java</exclude>
                                 <exclude>src/main/java/org/exist/dom/persistent/Match.java</exclude>
                                 <exclude>src/main/java/org/exist/dom/persistent/NewArrayNodeSet.java</exclude>

exist-core/src/main/java/org/exist/dom/persistent/EmptyNodeSet.java

@@ -1,4 +1,28 @@
 /*
+ * Elemental
+ * Copyright (C) 2024, Evolved Binary Ltd
+ *
+ * admin@evolvedbinary.com
+ * https://www.evolvedbinary.com | https://www.elemental.xyz
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; version 2.1.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * NOTE: Parts of this file contain code from 'The eXist-db Authors'.
+ *       The original license header is included below.
+ *
+ * =====================================================================
+ *
  * eXist-db Open Source Native XML Database
  * Copyright (C) 2001 The eXist-db Authors
  *
@@ -67,10 +91,12 @@ public boolean hasOne() {
 
     @Override
     public void add(final NodeProxy proxy) {
+        throw new IllegalStateException("Cannot add a NodeProxy to an EmptyNodeSet because it is immutable");
     }
 
     @Override
     public void addAll(final NodeSet other) {
+        throw new IllegalStateException("Cannot add a NodeSet to an EmptyNodeSet because it is immutable");
     }
 
     @Override