Fix to string for maps with values of type Empty

The MAX_KEY inserted into the tree had a value of one byte. This
triggered an assertion for maps with values of type Empty, because they
expected values to be empty.
Fixed by using an empty array for the value of the MAX_KEY.

block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java

@@ -179,6 +179,66 @@ public class PersistentMapTest {
 
 		}
 	}
+	@Test(invocationCount = 1)
+	public void testManyEmptyValues() throws Exception {
+		final Path file = dataDirectory.resolve("map.db");
+		final var insertedValues = new HashMap<Long, Empty>();
+		
+		final SecureRandom rnd = new SecureRandom();
+		rnd.setSeed(1);
+		
+		try (final PersistentMap<Long, Empty> map = new PersistentMap<>(file, PersistentMap.LONG_CODER,
+				PersistentMap.EMPTY_ENCODER)) {
+			
+			for (int i = 0; i < 1500; i++) {
+				// System.out.println("\n\ninserting: " + i);
+				
+				final Long key = (long) (rnd.nextGaussian() * Integer.MAX_VALUE);
+				final Empty value = Empty.INSTANCE;
+				Assert.assertNull(map.getValue(key));
+				
+				Assert.assertNull(map.putValue(key, value));
+				
+				insertedValues.put(key, value);
+				
+				// map.print();
+				
+				final boolean failEarly = false;
+				if (failEarly) {
+					for (final var entry : insertedValues.entrySet()) {
+						final Empty actualValue = map.getValue(entry.getKey());
+						
+						if (!Objects.equals(actualValue, entry.getValue())) {
+							map.print();
+						}
+						
+						Assert.assertEquals(actualValue, entry.getValue(),
+								"value for key " + entry.getKey() + " in the " + i + "th iteration");
+					}
+				}
+			}
+		}
+		
+		try (final PersistentMap<Long, Empty> map = new PersistentMap<>(file, PersistentMap.LONG_CODER,
+				PersistentMap.EMPTY_ENCODER)) {
+			map.print();
+			final AtomicInteger counter = new AtomicInteger();
+			map.visitNodeEntriesPreOrder(
+					(node, parentNode, nodeEntry, depth) -> counter.addAndGet(nodeEntry.isInnerNode() ? 1 : 0));
+			
+			Assert.assertEquals(counter.get(), 4,
+					"number of nodes should be small. Any number larger than 4 indicates, "
+							+ "that new inner nodes are created even though the existing inner "
+							+ "nodes could hold the values");
+			
+			for (final var entry : insertedValues.entrySet()) {
+				final Empty actualValue = map.getValue(entry.getKey());
+				Assert.assertEquals(actualValue, entry.getValue(),
+						"value for key " + entry.getKey() + " after all iterations");
+			}
+			
+		}
+	}
 
 	@Test(invocationCount = 1)
 	public void testEasyValues() throws Exception {