[MRG] do not try to complete when X_source is empty

[antoinecollas]

When doing source_target_merge(X_source, X_target), if one of the two X_source (or X_target) is empty then it infers what should be its default value based on the other one using the _find_y_type function.
This PR avoids this by checking the length of sample_domain for the empty X_source or X_target. If the length is 0, then it skips the imputation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.28%. Comparing base (5723185) to head (ae27803).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #280   +/-   ##
=======================================
  Coverage   97.28%   97.28%           
=======================================
  Files          61       61           
  Lines        6375     6382    +7     
=======================================
+ Hits         6202     6209    +7     
  Misses        173      173           

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