Fix ModelInsights for xgboost

[kinfaikan]

Related issues
OpWorkflowModel::modelInsights fails for xgboost because feature scores are not extracted correctly

Describe the proposed solution
Convert feature score map to a vector in ModelInsights::getModelInsights
Handle null stage param values in ModelInsights::getStageInfo
Do not create a table when no validation results is found

Describe alternatives you've considered
NA

Additional context
NA

[salesforce-cla]

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[tovbinm]

1. why use Vector as return type for if you are doing to toArray anyways?
2. should we have a default value featureVectorSize: Option[Int] = None?

[tovbinm]

@kinfaikan ^

[codecov]

Codecov Report

Merging #170 into master will increase coverage by 0.04%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   86.25%   86.29%   +0.04%     
==========================================
  Files         305      305              
  Lines        9950     9963      +13     
  Branches      319      549     +230     
==========================================
+ Hits         8582     8598      +16     
+ Misses       1368     1365       -3

Impacted Files	Coverage Δ
.../ml/dmlc/xgboost4j/scala/spark/XGBoostParams.scala	95% <100%> (+5%)	⬆️
...c/main/scala/com/salesforce/op/ModelInsights.scala	92.24% <93.33%> (+0.5%)	⬆️
...ages/impl/classification/OpXGBoostClassifier.scala	31.57% <0%> (+1.75%)	⬆️
...op/stages/impl/regression/OpXGBoostRegressor.scala	18.18% <0%> (+2.27%)	⬆️

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[leahmcguire]

lgtm

[salesforce-cla]

Thanks for the contribution! It looks like @kinfaikan is an internal user so signing the CLA is not required. However, we need to confirm this.