fix DataSchema thread-safe issue

[walterddr]

Observed some NPE occuring in multistage engine for stored column data type.

this is b/c the data schema object were created during distributed stage planner to create the operator chain

• it is shared among multiple threads (operator chain is executed in multi-thread fashion) and upstream/downstream operator (most operators required to know its upstream data schema)
• however, due to a single data schema object, multiple thread could've accessed at the same time to gather stored column data type.
• the check-null, then create empty array, fill in each position individual could cause race-condition.

[codecov-commenter]

Codecov Report

Merging #9619 (1a51b55) into master (209c060) will increase coverage by 3.47%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #9619      +/-   ##
============================================
+ Coverage     60.33%   63.80%   +3.47%     
- Complexity     5148     5299     +151     
============================================
  Files          1926     1890      -36     
  Lines        103209   101451    -1758     
  Branches      15670    15467     -203     
============================================
+ Hits          62266    64733    +2467     
+ Misses        36271    31983    -4288     
- Partials       4672     4735      +63     

Flag	Coverage Δ
integration2	?
unittests1	67.32% <100.00%> (+0.02%)	⬆️
unittests2	15.63% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...java/org/apache/pinot/common/utils/DataSchema.java	72.80% <100.00%> (-3.72%)	⬇️
...t/core/query/reduce/SelectionDataTableReducer.java	69.04% <100.00%> (-2.39%)	⬇️
...va/org/apache/pinot/core/routing/RoutingTable.java	0.00% <0.00%> (-100.00%)	⬇️
...va/org/apache/pinot/common/config/NettyConfig.java	0.00% <0.00%> (-100.00%)	⬇️
...a/org/apache/pinot/common/metrics/MinionMeter.java	0.00% <0.00%> (-100.00%)	⬇️
...g/apache/pinot/common/metrics/ControllerMeter.java	0.00% <0.00%> (-100.00%)	⬇️
.../apache/pinot/common/metrics/BrokerQueryPhase.java	0.00% <0.00%> (-100.00%)	⬇️
.../apache/pinot/common/metrics/MinionQueryPhase.java	0.00% <0.00%> (-100.00%)	⬇️
...ache/pinot/server/access/AccessControlFactory.java	0.00% <0.00%> (-100.00%)	⬇️
...he/pinot/common/messages/SegmentReloadMessage.java	0.00% <0.00%> (-100.00%)	⬇️
... and 603 more

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[agavra]

nice! elegant fix - we may want to leave some comments inline explaining that this is necessary for thread-safety so it doesn't get refactored away