NullPointerException while running invalid queries.

Running the following queries against the sample baseballStates table in Pinot query console causes NPE stack trace to show up in console:

**select playerName, percentile(homeRuns, 80) from baseballStats limit 10**
```
[
{
"errorCode": 200,
"message": "QueryExecutionError:
java.lang.NullPointerException
at org.apache.pinot.core.plan.maker.InstancePlanMakerImplV2.isFitForMetadataBasedPlan(InstancePlanMakerImplV2.java:193)
at org.apache.pinot.core.plan.maker.InstancePlanMakerImplV2.makeSegmentPlanNode(InstancePlanMakerImplV2.java:145)
at org.apache.pinot.core.plan.maker.InstancePlanMakerImplV2.makeInstancePlan(InstancePlanMakerImplV2.java:117)
at org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:290)
at org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:215)
at org.apache.pinot.core.query.executor.QueryExecutor.processQuery(QueryExecutor.java:60)
at org.apache.pinot.core.query.scheduler.QueryScheduler.processQueryAndSerialize(QueryScheduler.java:157)
at org.apache.pinot.core.query.scheduler.QueryScheduler.lambda$createQueryFutureTask$0(QueryScheduler.java:141)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:111)
at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:58)
at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:75)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)"
}
]
```

**select playerName, percentile50mv(homeRuns) from baseballStats group by playerName limit 100**
```
[
{
"errorCode": 200,
"message": "QueryExecutionError:
java.lang.NullPointerException
at org.apache.pinot.core.operator.combine.GroupByOrderByCombineOperator.getNextBlock(GroupByOrderByCombineOperator.java:247)
at org.apache.pinot.core.operator.combine.GroupByOrderByCombineOperator.getNextBlock(GroupByOrderByCombineOperator.java:62)
at org.apache.pinot.core.operator.BaseOperator.nextBlock(BaseOperator.java:49)
at org.apache.pinot.core.operator.InstanceResponseOperator.getNextBlock(InstanceResponseOperator.java:37)
at org.apache.pinot.core.operator.InstanceResponseOperator.getNextBlock(InstanceResponseOperator.java:26)
at org.apache.pinot.core.operator.BaseOperator.nextBlock(BaseOperator.java:49)
at org.apache.pinot.core.plan.GlobalPlanImplV0.execute(GlobalPlanImplV0.java:48)
at org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:294)
at org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:215)
at org.apache.pinot.core.query.executor.QueryExecutor.processQuery(QueryExecutor.java:60)
at org.apache.pinot.core.query.scheduler.QueryScheduler.processQueryAndSerialize(QueryScheduler.java:157)
at org.apache.pinot.core.query.scheduler.QueryScheduler.lambda$createQueryFutureTask$0(QueryScheduler.java:141)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)"
}
]
```

Start with InstancePlanMakerImplV2.isFitForMetadataBasedPlan at line 193 and GroupByOrderByCombineOperator.getNextBlock at line 247, using the two baseballStats queries as reproductions. Trace the null values through the query planning and group-by execution paths. Done means both queries complete without exposing a NullPointerException stack trace in the Pinot query console.

Written by the indexing model from the issue text.