Replace empty XGBoost feature importance with a vector of zeros

[TuanNguyen27]

Related issues A successfully trained XGBoost model could return an empty feature importance vector when the features have zero signal w.r.t the label. This behavior will fail getModelContributions via this line.

require(featureScore.nonEmpty, "Feature score map is empty")

Describe the proposed solution In this case, we will return a feature contribution vector of 0's so that it matches the behavior of other models.

[codecov[bot]]

Codecov Report

Merging #487 into master will decrease coverage by 5.59%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
- Coverage   87.02%   81.43%   -5.60%     
==========================================
  Files         345      345              
  Lines       11683    11687       +4     
  Branches      387      377      -10     
==========================================
- Hits        10167     9517     -650     
- Misses       1516     2170     +654     

Impacted Files	Coverage Δ
...c/main/scala/com/salesforce/op/ModelInsights.scala	91.88% <33.33%> (-1.21%)	:arrow_down:
...ala/com/salesforce/op/utils/tuples/RichTuple.scala	0.00% <0.00%> (-100.00%)	:arrow_down:
...alesforce/op/aggregators/TimeBasedAggregator.scala	0.00% <0.00%> (-100.00%)	:arrow_down:
...stages/impl/feature/TimePeriodMapTransformer.scala	0.00% <0.00%> (-100.00%)	:arrow_down:
...e/op/stages/impl/insights/RecordInsightsCorr.scala	0.00% <0.00%> (-98.25%)	:arrow_down:
utils/src/main/scala/com/salesforce/op/UID.scala	0.00% <0.00%> (-91.67%)	:arrow_down:
...op/stages/impl/preparators/MinVarianceFilter.scala	0.00% <0.00%> (-91.31%)	:arrow_down:
...es/src/main/scala/com/salesforce/op/OpParams.scala	0.00% <0.00%> (-89.80%)	:arrow_down:
...ala/com/salesforce/op/stages/SparkStageParam.scala	0.00% <0.00%> (-77.42%)	:arrow_down:
...a/com/salesforce/op/utils/spark/RichMetadata.scala	15.78% <0.00%> (-73.69%)	:arrow_down:
... and 47 more

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