refactor: use train data if test data is empty for a cluster

[sathvikbhagavan]

As we sample train data and test data, it can happen that test data in one of the clusters is empty that leads to spurious CI failures.

This approach uses train data for obtaining best model in a cluster if there is no test data available as a fallback instead of erroring.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4f069df) 65.41% compared to head (12a0529) 78.31%.

Files	Patch %	Lines
lib/SurrogatesMOE/src/SurrogatesMOE.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #466       +/-   ##
===========================================
+ Coverage   65.41%   78.31%   +12.89%     
===========================================
  Files          23       23               
  Lines        3146     3159       +13     
===========================================
+ Hits         2058     2474      +416     
+ Misses       1088      685      -403     

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