[multistage][bugfix] fix group-by without agg call plan failure

[walterddr]

group-by without agg call is basically a DISTINCT operation. we should allow this.

[codecov-commenter]

Codecov Report

Merging #9383 (af30a87) into master (987480b) will decrease coverage by 2.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #9383      +/-   ##
============================================
- Coverage     63.44%   61.16%   -2.28%     
+ Complexity     5069     4560     -509     
============================================
  Files          1832     1873      +41     
  Lines         98122   100039    +1917     
  Branches      15017    15238     +221     
============================================
- Hits          62250    61191    -1059     
- Misses        31283    34200    +2917     
- Partials       4589     4648      +59     

Flag	Coverage Δ
integration1	25.96% <0.00%> (?)
unittests1	66.97% <100.00%> (+0.01%)	⬆️
unittests2	?

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Impacted Files	Coverage Δ
.../java/org/apache/pinot/query/QueryEnvironment.java	83.60% <100.00%> (+1.46%)	⬆️
...pinot/controller/recommender/io/ConfigManager.java	0.00% <0.00%> (-100.00%)	⬇️
.../org/apache/pinot/client/AggregationResultSet.java	0.00% <0.00%> (-100.00%)	⬇️
...ntroller/recommender/rules/impl/JsonIndexRule.java	0.00% <0.00%> (-100.00%)	⬇️
...troller/recommender/io/metadata/FieldMetadata.java	0.00% <0.00%> (-100.00%)	⬇️
...roller/recommender/rules/impl/BloomFilterRule.java	0.00% <0.00%> (-100.00%)	⬇️
...oller/api/resources/PinotControllerAppConfigs.java	0.00% <0.00%> (-100.00%)	⬇️
...ler/recommender/data/generator/BytesGenerator.java	0.00% <0.00%> (-100.00%)	⬇️
...er/recommender/io/metadata/SchemaWithMetaData.java	0.00% <0.00%> (-100.00%)	⬇️
...r/recommender/rules/impl/AggregateMetricsRule.java	0.00% <0.00%> (-100.00%)	⬇️
... and 558 more

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