slashing: absent val to DEBUG

[tac0turtle]

Description

Change absent validator info log to debug.

closes: #XXXX

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #8752 (1cef99e) into master (a193522) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8752   +/-   ##
=======================================
  Coverage   61.44%   61.44%           
=======================================
  Files         673      673           
  Lines       38361    38361           
=======================================
  Hits        23571    23571           
  Misses      12309    12309           
  Partials     2481     2481           

Impacted Files	Coverage Δ
x/slashing/keeper/infractions.go	83.56% <100.00%> (ø)