Add two test cases for init DiffractionObject with empty xarray and yarray values

[bobleesj]

Closes #241

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b0b6676) to head (b865a8e).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #284   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines          405       408    +3     
=========================================
+ Hits           405       408    +3     

Files with missing lines	Coverage Δ
tests/test_diffraction_objects.py	100.00% <100.00%> (ø)

[bobleesj]

@sbillinge ready for review