Flaky-test: org.apache.pulsar.client.impl.schema.SchemaInfoTest.testSchemaInfoToString

SchemaInfoTest.testSchemaInfoToString is flaky. It fails sporadically.

[example failure](https://github.com/apache/pulsar/runs/7234112994?check_suite_focus=true)

```
Error: Tests run: 9, Failures: 1, Errors: 0, Skipped: 5, Time elapsed: 1.121 s <<< FAILURE! - in org.apache.pulsar.client.impl.schema.SchemaInfoTest
Error: testSchemaInfoToString(org.apache.pulsar.client.impl.schema.SchemaInfoTest) Time elapsed: 0.01 s <<< FAILURE!
java.lang.NullPointerException: Cannot invoke "Object.toString()" because "si" is null
at org.apache.pulsar.client.impl.schema.SchemaInfoTest.testSchemaInfoToString(SchemaInfoTest.java:284)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
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:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
```

Start with SchemaInfoTest.java, specifically testSchemaInfoToString around line 284, and review the test setup that leaves si null in the reported failure. Run the test repeatedly to reproduce the intermittent NullPointerException; done means the test no longer fails sporadically and still verifies SchemaInfo.toString().

Written by the indexing model from the issue text.