[improve][client]Add null check for Pulsar client clock configuration (apache#24848)

(cherry picked from commit 54db7f2)
(cherry picked from commit fb1b2a5)

Author: 3pacccccc

pulsar-broker/src/test/java/org/apache/pulsar/client/api/SimpleProducerConsumerTest.java

@@ -319,6 +319,17 @@ public void testSlashSubscriptionName() throws Exception {
         });
     }
 
+    @Test(timeOut = 5000)
+    public void pulsarClientClockCheckTest() {
+        assertThatThrownBy(
+                () -> PulsarClient.builder()
+                    .serviceUrl(lookupUrl.toString())
+                    .clock(null)
+                    .build()
+        ).isInstanceOf(IllegalArgumentException.class)
+                .hasMessage("clock must not be null");
+    }
+
     @Test(timeOut = 100000)
     public void testPublishTimestampBatchEnabled() throws Exception {
 

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ClientBuilderImpl.java

@@ -380,6 +380,7 @@ public ClientBuilder memoryLimit(long memoryLimit, SizeUnit unit) {
 
     @Override
     public ClientBuilder clock(Clock clock) {
+        checkArgument(clock != null, "clock must not be null");
         conf.setClock(clock);
         return this;
     }