Flaky-test: org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest.allSchemasShouldSupportNull and allSchemasShouldRoundtripInput

```
java.lang.NullPointerException: NPE when using schema null : null
at org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest.allSchemasShouldRoundtripInput(PrimitiveSchemaTest.java:132)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
at org.testng.internal.InvokeMethodRunnable.runOne(InvokeMethodRunnable.java:45)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:73)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:11)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)
```

output:

> Starting test [TestClass name=class org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest].allSchemasShouldRoundtripInput([{null=[00:18:23.823175, 22:18:23.823181], org.apache.pulsar.client.impl.schema.ByteSchema@2b4829aa=[-1, 0], org.apache.pulsar.client.impl.schema.DateSchema@32d22650=[2021-06-23, 2021-06-23], org.apache.pulsar.client.impl.schema.LocalDateSchema@33a71d23=[2021-06-23, 2021-06-21], org.apache.pulsar.client.impl.schema.BytesSchema@2fa879ed=[[B@9880a15], org.apache.pulsar.client.impl.schema.ShortSchema@794d28a3=[32767, -32768], org.apache.pulsar.client.impl.schema.TimestampSchema@4f6fd101=[2021-06-23 00:18:13.82, 2021-06-23 00:18:23.82], org.apache.pulsar.client.impl.schema.DoubleSchema@a99c42c=[5678567.12312, -5678567.12341], org.apache.pulsar.client.impl.schema.ByteBufferSchema@2b2b7e3c=[java.nio.HeapByteBuffer[pos=9 lim=10 cap=10]], org.apache.pulsar.client.impl.schema.StringSchema@aee05f4=[my string], org.apache.pulsar.client.impl.schema.TimeSchema@135064ea=[00:18:13, 00:18:23], org.apache.pulsar.client.impl.schema.InstantSchema@106459d9=[2021-06-23T00:18:23.820925Z, 2021-06-22T23:55:23.820928Z], org.apache.pulsar.client.impl.schema.FloatSchema@1a01ffff=[5678567.0, -5678567.0], org.apache.pulsar.client.impl.schema.BooleanSchema@2bcb1414=[false, true], org.apache.pulsar.client.impl.schema.IntSchema@a55e82a=[423412424, -41243432], org.apache.pulsar.client.impl.schema.LongSchema@70ee9d31=[922337203685477580, -922337203685477581], org.apache.pulsar.client.impl.schema.LocalDateTimeSchema@12166229=[2021-06-23T00:18:23.823191, 2021-06-21T00:18:23.823199, 2021-04-14T00:18:23.823209]}])-------
> 00:18:23.964 [TestNG-method=allSchemasShouldRoundtripInput-1:org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest@122] INFO org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest - Test schema null
> 00:18:23.964 [TestNG-method=allSchemasShouldRoundtripInput-1:org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest@124] INFO org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest - Encode : 00:18:23.823175

Start with org.apache.pulsar.client.impl.schema.PrimitiveSchemaTest and run allSchemasShouldSupportNull and allSchemasShouldRoundtripInput, focusing on the failure at line 132. Reproduce the null-schema NPE and compare the test output for the two cases. Done means both tests pass reliably without the reported flaky failure.

Written by the indexing model from the issue text.