Fix crash in unix users plugins if GID or UID is empty

[wbi-ocd]

This PR fix an issue while processing /etc/passwd entry where gid or uid id empty.
This kind OS issue could also be fixed more generally in flow, by checking if entry is empty before coercing into an int, but I'm not sure if this would be the expected comportment for all entries.
Also modify error handler to backslash replace, to prevent crash in case file contains non utf-8 characters.

fix #995

[wbi-ocd]

@Horofic, changes have been applied. I have also completed test with the new entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.76%. Comparing base (e7d007a) to head (d025884).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #996   +/-   ##
=======================================
  Coverage   77.76%   77.76%           
=======================================
  Files         327      327           
  Lines       28654    28654           
=======================================
  Hits        22282    22282           
  Misses       6372     6372           

Flag	Coverage Δ
unittests	77.76% <100.00%> (ø)

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