Support for tombstones (null value in messages) does not work

**Describe the bug**

The solution provided by #7139 to the BUG #4803 *does not work*.

* When trying to read a message with `null` value a `NullPointerException` is thrown in other part of the code.
```
msg = consumer.receive(timeoutMillis, TimeUnit.MILLISECONDS));

java.lang.NullPointerException
at org.apache.pulsar.client.impl.ConsumerStatsRecorderImpl.updateNumMsgsReceived(ConsumerStatsRecorderImpl.java:169)
at org.apache.pulsar.client.impl.ConsumerImpl.messageProcessed(ConsumerImpl.java:1423)
at org.apache.pulsar.client.impl.ConsumerImpl.internalReceive(ConsumerImpl.java:431)
at org.apache.pulsar.client.impl.ConsumerBase.receive(ConsumerBase.java:175)
...
```
* It should not be required to explicitly indicate a null value to producer (only-key values should work just just fine). Exception thrown when working with implicit null value messages is EOFException in this case (the same before this a solution was provided).

**To Reproduce**
I have created a test project to reproduce these issues (`null` values implicitly and explictly set with both schema and schemaless consumer): [pulsar-tombstone-test](https://github.com/fmiguelez/pulsar-tombstone-test)

Read README.md to reproduce it.

**Expected behavior**
Tombstones (`null` values in mesages with our without schema but with key) should be supported whether you indicate an schema or not and whether you explicitly indicate a `null` value or not (implicit `null` value). All tests should pass in the example project.

```
Producer schemalessProducer = client.newProducer(Schema.BYTES).topic(TOPIC).messageRoutingMode(MessageRoutingMode.SinglePartition).create();

Consumer schemalessConsumer = client.newConsumer(Schema.BYTES).topic(TOPIC).subscriptionName("test")
.subscriptionInitialPosition(SubscriptionInitialPosition.Earliest).subscribe();

// Implicit tombstone without schema
schemalessProducer.key("1").send();

// Explicit tombstone (the one supposedly to work)
schemalessProducer.key("2").value(null).send();

Message implicitTombstone = schemalessReceiver.receive(2, TimeUnit.SECONDS).getValue());
Message explictitTombstone = schemalessReceiver.receive(2, TimeUnit.SECONDS).getValue())

System.out.println(String.format("Implicit tombstone: {key=%s, value=%s}", implicitTombstone.getKey(), implicitTombstone.getValue()));
System.out.println(String.format("Explicit tombstone: {key=%s, value=%s}", implicitTombstone.getKey(), implicitTombstone.getValue()));
```

```
Producer schemaProducer = client.newProducer(Schema.AVRO(DummyObject.class)).topic(TOPIC).messageRoutingMode(MessageRoutingMode.SinglePartition).create();

Consumer schemaConsumer = client.newConsumer(Schema.AVRO(DummyObject.class)).topic(TOPIC).subscriptionName("test")
.subscriptionInitialPosition(SubscriptionInitialPosition.Earliest).subscribe();

// Implicit tombstone without schema
schemaProducer.key("1").send();

// Explicit tombstone (the one supposedly to work)
schemaProducer.key("2").value(null).send();

Message implicitTombstone = schemalessReceiver.receive(2, TimeUnit.SECONDS).getValue());
Message explictitTombstone = schemalessReceiver.receive(2, TimeUnit.SECONDS).getValue())

System.out.println(String.format("Implicit tombstone: {key=%s, value=%s}", implicitTombstone.getKey(), implicitTombstone.getValue()));
System.out.println(String.format("Explicit tombstone: {key=%s, value=%s}", implicitTombstone.getKey(), implicitTombstone.getValue()));
```

**Screenshots**

**Desktop (please complete the following information):**
- Windows 10 with Docker Deskto to run Pulsar containers

**Additional context**

Start with the README.md in the linked pulsar-tombstone-test reproduction and trace the reported failure through ConsumerStatsRecorderImpl.java, ConsumerImpl.java, and ConsumerBase.java. Reproduce implicit and explicit tombstones with schema and schemaless consumers; done means keyed null messages are supported in all four cases and the reproduction tests pass.

Written by the indexing model from the issue text.