logs: Add empty value for KeyValue

[XSAM]

resolves #4953

I find log.Empty is useful when using log.MapValue. For that, we can have a consistent user experience while adding key values to the map.

log.MapValue(
	log.Slice("l", log.IntValue(3), log.StringValue("foo")),
	log.Bytes("b", []byte{3, 5, 7}),
	log.Empty("e"),
)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.2%. Comparing base (42c1708) to head (4a5e0e3).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5076   +/-   ##
=====================================
  Coverage   83.2%   83.2%           
=====================================
  Files        243     243           
  Lines      15908   15910    +2     
=====================================
+ Hits       13245   13247    +2     
  Misses      2375    2375           
  Partials     288     288           

Files	Coverage Δ
log/keyvalue.go	97.6% <100.0%> (+<0.1%)	⬆️

[pellared]

LGTM 👍 I left just a few comments that should be resolved.