[fix][broker] Fix cluster level OffloadedReadPriority to bookkeeper-first does not work (apache#24151)

### Motivation
When we set cluster level `managedLedgerDataReadPriority` to `bookkeeper-first` in conf/broker.conf
https://github.com/apache/pulsar/blob/f2618c15bb5a9f3fcb577068584df5d0e2e4f335/conf/broker.conf#L1262-L1264

Then use the command `bin/pulsar-admin namespaces set-offload-threshold -s 0 <namespace>` to set the offload threshold in namespace level to trigger topic offload automatically.

However, the broker side will create an empty OffloadPolicy when setting the offload threshold
https://github.com/apache/pulsar/blob/f2618c15bb5a9f3fcb577068584df5d0e2e4f335/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/NamespacesBase.java#L2174-L2176

However, the empty OffloadPolicy will set the `OffloadedReadPriority` to `TIERED_STORAGE_FIRST` in namespace level.
https://github.com/apache/pulsar/blob/f2618c15bb5a9f3fcb577068584df5d0e2e4f335/pulsar-common/src/main/java/org/apache/pulsar/common/policies/data/OffloadPoliciesImpl.java#L98-L99

The namespace level offload policy has higher priority than broker level offload policy, and it makes setting broker-level `OffloadedReadPriority` to `bookkeeper-first` doesn't work

### Modifications

Change OffloadPolicy's default OffloadedReadPriority to `null` to allow broker-level offload policy can take affect.

(cherry picked from commit 153fce9)
(cherry picked from commit 02257c8)

Author: hangc0276

pulsar-common/src/main/java/org/apache/pulsar/common/policies/data/OffloadPoliciesImpl.java

@@ -94,8 +94,7 @@ public class OffloadPoliciesImpl implements Serializable, OffloadPolicies {
             "managedLedgerOffloadAutoTriggerSizeThresholdBytes";
     public static final String DELETION_LAG_NAME_IN_CONF_FILE = "managedLedgerOffloadDeletionLagMs";
     public static final String DATA_READ_PRIORITY_NAME_IN_CONF_FILE = "managedLedgerDataReadPriority";
-    public static final OffloadedReadPriority DEFAULT_OFFLOADED_READ_PRIORITY =
-            OffloadedReadPriority.TIERED_STORAGE_FIRST;
+    public static final OffloadedReadPriority DEFAULT_OFFLOADED_READ_PRIORITY = null;
 
     // common config
     @Configuration

pulsar-common/src/test/java/org/apache/pulsar/common/policies/data/OffloadPoliciesTest.java

@@ -20,6 +20,8 @@
 
 import static org.apache.pulsar.common.policies.data.OffloadPoliciesImpl.EXTRA_CONFIG_PREFIX;
 import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertNull;
+
 import java.io.DataInputStream;
 import java.io.File;
 import java.io.IOException;
@@ -472,4 +474,13 @@ public void testToProperties() throws Exception {
         offloadPolicies.setManagedLedgerExtraConfigurations(extraConfiguration); 
         assertEquals(offloadPolicies, OffloadPoliciesImpl.create(offloadPolicies.toProperties()));
     }
+
+    @Test
+    public void testDefaultOffloadPolicies() {
+        OffloadPoliciesImpl offloadPolicies = new OffloadPoliciesImpl();
+        assertNull(offloadPolicies.getManagedLedgerOffloadedReadPriority());
+        assertNull(offloadPolicies.getManagedLedgerOffloadDeletionLagInMillis());
+        assertNull(offloadPolicies.getManagedLedgerOffloadThresholdInSeconds());
+        assertNull(offloadPolicies.getManagedLedgerOffloadThresholdInBytes());
+    }
 }