fix: Serilog not handling null strings

[glennawatson]

housekeeping: Update all targets to net462, net461 no longer supported

[codecov]

Codecov Report

Merging #925 (f603ed1) into main (476e73c) will decrease coverage by 0.10%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main     #925      +/-   ##
==========================================
- Coverage   75.13%   75.03%   -0.11%     
==========================================
  Files          91       91              
  Lines        4883     4883              
==========================================
- Hits         3669     3664       -5     
- Misses       1214     1219       +5     

Impacted Files	Coverage Δ
...nsions.Logging/MicrosoftExtensionsLoggingLogger.cs	86.95% <ø> (ø)
.../Splat.Raygun/RaygunFeatureUsageTrackingSession.cs	90.69% <ø> (ø)
src/Splat.Serilog/SerilogFullLogger.cs	74.78% <60.00%> (ø)
src/Splat/Logging/WrappingPrefixLogger.cs	64.70% <0.00%> (-5.89%)	⬇️
...ceMonitoring/DefaultFeatureUsageTrackingSession.cs	68.00% <0.00%> (-4.00%)	⬇️
src/Splat/Logging/WrappingFullLogger.cs	64.28% <0.00%> (-1.30%)	⬇️
src/Splat/Logging/FullLoggerExtensions.cs	27.61% <0.00%> (-0.96%)	⬇️

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[github-actions]

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