fix: panic on aggregate without GROUP BY clause

[laysakura]

fixes: #126

Also, GROUP BY clause gets to take multiple elements.

[codecov]

Codecov Report

Merging #129 (37b2536) into main (cccad2d) will increase coverage by 0.13%.
The diff coverage is 97.52%.

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   87.30%   87.43%   +0.13%     
==========================================
  Files         201      202       +1     
  Lines       12104    12259     +155     
==========================================
+ Hits        10567    10719     +152     
- Misses       1537     1540       +3     

Impacted Files	Coverage Δ
.../pipeline/pump_model/window_operation_parameter.rs	100.00% <ø> (ø)
...ringql-core/src/sql_processor/sql_parser/syntax.rs	50.00% <ø> (ø)
springql-core/src/sql_processor/query_planner.rs	95.18% <82.35%> (+0.18%)	⬆️
...e/src/sql_processor/sql_parser/pest_parser_impl.rs	87.72% <92.85%> (+0.09%)	⬆️
..._executor/task/window/panes/pane/aggregate_pane.rs	95.72% <97.72%> (+1.28%)	⬆️
...pump_model/window_operation_parameter/aggregate.rs	100.00% <100.00%> (ø)
..._planner/select_syntax_analyzer/group_aggregate.rs	100.00% <100.00%> (ø)
...ngine/autonomous_executor/task/window/aggregate.rs	98.02% <100.00%> (ø)
...am_engine/autonomous_executor/task/window/panes.rs	98.05% <100.00%> (+0.01%)	⬆️
springql-core/tests/feat_aggregation.rs	100.00% <100.00%> (ø)
... and 3 more

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