ut: add null test

[wanghan-iapcm]

test the cases:

1. system only has one atom
2. system has two atoms that are far away from each other.

In each cases, the force and virial predictions should be zero and energy should be a valid float.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.59%. Comparing base (f4abe12) to head (a7ec2d0).
Report is 8 commits behind head on devel.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3391      +/-   ##
==========================================
+ Coverage   76.57%   76.59%   +0.02%     
==========================================
  Files         420      420              
  Lines       35682    35754      +72     
  Branches     1620     1620              
==========================================
+ Hits        27323    27387      +64     
- Misses       7491     7499       +8     
  Partials      868      868              

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