Fix HashMapIdObject.toString() on an empty map

base/src/main/java/org/eclipse/serializer/collections/HashMapIdObject.java

@@ -296,6 +296,11 @@ public final int iterateValues(final Consumer<? super E> procedure)
 	@Override
 	public final String toString()
 	{
+		if(this.size == 0)
+		{
+			return "{}";
+		}
+
 		final VarString vc = VarString.New().add('{');
 		for(Entry<E> entry : this.hashSlots)
 		{

integration-tests/src/test/java/org/eclipse/serializer/collections/HashMapIdObjectTest.java

@@ -0,0 +1,46 @@
+package org.eclipse.serializer.collections;
+
+/*-
+ * #%L
+ * Eclipse Serializer Integration Tests
+ * %%
+ * Copyright (C) 2023 - 2026 MicroStream Software
+ * %%
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ * #L%
+ */
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class HashMapIdObjectTest
+{
+	@Test
+	void toString_emptyMap()
+	{
+		// regression: threw StringIndexOutOfBoundsException
+		assertEquals("{}", HashMapIdObject.New().toString());
+	}
+
+	@Test
+	void toString_singleEntry()
+	{
+		final HashMapIdObject<String> map = HashMapIdObject.New();
+		map.add(1L, "a");
+		assertEquals("{1 -> a}", map.toString());
+	}
+
+	@Test
+	void toString_afterClear()
+	{
+		final HashMapIdObject<String> map = HashMapIdObject.New();
+		map.add(1L, "a");
+		map.clear();
+		assertEquals("{}", map.toString());
+	}
+}