free: add unit to zero in human mode

[cakebaker]

When using --human, values that are zero are currently shown without a unit in our implementation whereas the original free displays such values as 0B. This PR adds the missing unit to such values.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.76%. Comparing base (3bd07df) to head (ffbb6da).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/free/src/free.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #252   +/-   ##
=======================================
  Coverage   60.75%   60.76%           
=======================================
  Files          44       44           
  Lines        4495     4496    +1     
  Branches      687      688    +1     
=======================================
+ Hits         2731     2732    +1     
- Misses       1608     1609    +1     
+ Partials      156      155    -1     

Flag	Coverage Δ
macos_latest	46.17% <0.00%> (-0.02%)	⬇️
ubuntu_latest	58.58% <0.00%> (+0.05%)	⬆️
windows_latest	0.05% <0.00%> (-0.01%)	⬇️

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