handle pending minion tasks properly when getting the task progress status

[klsince]

bugfix: when subtasks are waiting in Helix, the get task progress endpoint failed with NPE as those pending subtasks don't have assigned workers yet. This PR handles pending tasks properly.

Validated that the Pinot UI tab Minion Task Manager could show the task progress properly now even with pending subtasks.

[codecov-commenter]

Codecov Report

Merging #9911 (ac089c8) into master (f743ff1) will decrease coverage by 37.83%.
The diff coverage is 95.45%.

@@              Coverage Diff              @@
##             master    #9911       +/-   ##
=============================================
- Coverage     70.34%   32.51%   -37.84%     
+ Complexity     5656      191     -5465     
=============================================
  Files          1989     1989               
  Lines        107181   107187        +6     
  Branches      16290    16293        +3     
=============================================
- Hits          75400    34849    -40551     
- Misses        26515    69289    +42774     
+ Partials       5266     3049     -2217     

Flag	Coverage Δ
integration1	25.05% <0.00%> (-0.06%)	⬇️
integration2	?
unittests1	?
unittests2	13.62% <95.45%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
...lix/core/minion/PinotHelixTaskResourceManager.java	43.81% <95.45%> (+3.10%)	⬆️
...in/java/org/apache/pinot/spi/utils/BytesUtils.java	0.00% <0.00%> (-100.00%)	⬇️
...java/org/apache/pinot/spi/trace/BaseRecording.java	0.00% <0.00%> (-100.00%)	⬇️
...java/org/apache/pinot/spi/trace/NoOpRecording.java	0.00% <0.00%> (-100.00%)	⬇️
...ava/org/apache/pinot/spi/config/table/FSTType.java	0.00% <0.00%> (-100.00%)	⬇️
...ava/org/apache/pinot/spi/config/user/RoleType.java	0.00% <0.00%> (-100.00%)	⬇️
...ava/org/apache/pinot/spi/data/MetricFieldSpec.java	0.00% <0.00%> (-100.00%)	⬇️
...ava/org/apache/pinot/spi/stream/StreamMessage.java	0.00% <0.00%> (-100.00%)	⬇️
...java/org/apache/pinot/common/tier/TierFactory.java	0.00% <0.00%> (-100.00%)	⬇️
...a/org/apache/pinot/spi/config/table/TableType.java	0.00% <0.00%> (-100.00%)	⬇️
... and 1218 more

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[snleee]

Is it possible to add the testing case to cover this scenario?

[snleee]

LGTM. Thank you!