Translate empty Windows evtx value to None

[JSCU-CNI]

This PR replaces empty evtx values "-" to None in format_value. We also have added some example Security.evtx logs in the tests.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.34%. Comparing base (fe3ca7d) to head (696af01).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/plugins/os/windows/log/evtx.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1058   +/-   ##
=======================================
  Coverage   79.33%   79.34%           
=======================================
  Files         347      347           
  Lines       31012    31023   +11     
=======================================
+ Hits        24604    24614   +10     
- Misses       6408     6409    +1     

Flag	Coverage Δ
unittests	79.34% <93.75%> (+<0.01%)	⬆️

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