Test for presence of ack result message and simplify ProcessControllerAckResult API

[cmnbroad]

Fixes NPE reported in #7811.

[codecov]

Codecov Report

Merging #7816 (69be797) into master (9ae1fd8) will decrease coverage by 37.291%.
The diff coverage is 15.385%.

❗ Current head 69be797 differs from pull request most recent head c7832fb. Consider uploading reports for the commit c7832fb to get more accurate results

@@               Coverage Diff                @@
##              master     #7816        +/-   ##
================================================
- Coverage     86.954%   49.663%   -37.291%     
+ Complexity     36897     26502     -10395     
================================================
  Files           2214      2212         -2     
  Lines         173540    173384       -156     
  Branches       18736     18710        -26     
================================================
- Hits          150900     86108     -64792     
- Misses         16037     81594     +65557     
+ Partials        6603      5682       -921     

Impacted Files	Coverage Δ
...ls/runtime/ProcessControllerAckResultUnitTest.java	9.091% <9.091%> (ø)
...nder/utils/runtime/ProcessControllerAckResult.java	94.737% <50.000%> (+8.373%)	⬆️
...titute/hellbender/utils/UnvalidatingGenomeLoc.java	0.000% <0.000%> (-100.000%)	⬇️
...itute/hellbender/utils/locusiterator/LIBSTest.java	0.000% <0.000%> (-100.000%)	⬇️
...ellbender/engine/AlignmentAndReferenceContext.java	0.000% <0.000%> (-100.000%)	⬇️
...ools/copynumber/models/CopyRatioSimulatedData.java	0.000% <0.000%> (-100.000%)	⬇️
...ols/walkers/mutect/clustering/BinomialCluster.java	0.000% <0.000%> (-100.000%)	⬇️
...copynumber/models/AlleleFractionSimulatedData.java	0.000% <0.000%> (-100.000%)	⬇️
...otypecaller/DragstrPairHMMInputScoreImputator.java	0.000% <0.000%> (-100.000%)	⬇️
...urces/gencode/DataProviderForMuc16MnpFullData.java	0.000% <0.000%> (-99.952%)	⬇️
... and 690 more

[lbergelson]

@cmnbroad This looks good to me.