V3UpdateIndexException: Default value indices for column cannot be updated for V3 format segment.

Hello,

As discussed in this [slack thread](https://apache-pinot.slack.com/archives/C011C9JHN7R/p1621607719059200) there seems to be a bug in Pinot v0.7.1 where updating the default value of an existing dimension field of type String in a Schema will through an exception of this sort:
```
2021/05/21 14:32:34.378 ERROR [SegmentOnlineOfflineStateModelFactory$SegmentOnlineOfflineStateModel] [HelixTaskExecutor-message_handle_thread] Caught exception in state transition from OFFLINE -> ONLINE for resource: HitExecutionView_REALTIME, partition: HitExecutionView__12__21__20210520T1019Z
org.apache.pinot.core.segment.index.loader.V3UpdateIndexException: Default value indices for column: inputForUiControls cannot be updated for V3 format segment.
at org.apache.pinot.core.segment.index.loader.defaultcolumn.V3DefaultColumnHandler.updateDefaultColumn(V3DefaultColumnHandler.java:53) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.core.segment.index.loader.defaultcolumn.BaseDefaultColumnHandler.updateDefaultColumns(BaseDefaultColumnHandler.java:144) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.core.segment.index.loader.SegmentPreProcessor.process(SegmentPreProcessor.java:104) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.core.indexsegment.immutable.ImmutableSegmentLoader.load(ImmutableSegmentLoader.java:99) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.core.data.manager.realtime.RealtimeTableDataManager.addSegment(RealtimeTableDataManager.java:283) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.server.starter.helix.HelixInstanceDataManager.addRealtimeSegment(HelixInstanceDataManager.java:138) ~[pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.pinot.server.starter.helix.SegmentOnlineOfflineStateModelFactory$SegmentOnlineOfflineStateModel.onBecomeOnlineFromOffline(SegmentOnlineOfflineStateModelFactory.java:164) [pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at sun.reflect.GeneratedMethodAccessor48.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_282]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_282]
at org.apache.helix.messaging.handling.HelixStateTransitionHandler.invoke(HelixStateTransitionHandler.java:404) [pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.helix.messaging.handling.HelixStateTransitionHandler.handleMessage(HelixStateTransitionHandler.java:331) [pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.helix.messaging.handling.HelixTask.call(HelixTask.java:97) [pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at org.apache.helix.messaging.handling.HelixTask.call(HelixTask.java:49) [pinot-all-0.7.1-jar-with-dependencies.jar:0.7.1-afa4b252ab1c424ddd6c859bb305b2aa342b66ed]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_282]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_282]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_282]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_282]
```

The concrete change in question was from:
```
{
"name": "inputForUiControls",
"dataType": "STRING",
"maxLength": 2147483647
}
```
to:
```
{
"name": "inputForUiControls",
"dataType": "STRING",
"defaultNullValue": "{}",
"maxLength": 2147483647
}
```

This pinot instance already had existing segments in the servers with a backup in deep storage and as such I had triggered a segment reload.

Thank you.

Start with V3DefaultColumnHandler.updateDefaultColumn and BaseDefaultColumnHandler.updateDefaultColumns, then trace how SegmentPreProcessor.process handles a reloaded segment. Reproduce the schema change from no defaultNullValue to defaultNullValue on an existing STRING column and reload a V3 segment. Done means the segment loads successfully without V3UpdateIndexException.

Written by the indexing model from the issue text.