feat: add MsgEmpty to auth module

[wetcod]

Description

closes: #XXXX

• add MsgEmpty to auth module
• MsgEmpty doesn't do anything. Used to measure performance.

---

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 #176 (af3e597) into v2/develop (f9c3e64) will increase coverage by 29.65%.
The diff coverage is 61.11%.

@@               Coverage Diff               @@
##           v2/develop     #176       +/-   ##
===============================================
+ Coverage       23.44%   53.09%   +29.65%     
===============================================
  Files             765      688       -77     
  Lines          125253    49052    -76201     
===============================================
- Hits            29362    26046     -3316     
+ Misses          92040    20111    -71929     
+ Partials         3851     2895      -956     

Impacted Files	Coverage Δ
x/auth/types/keys.go	100.00% <ø> (ø)
x/auth/keeper/msg_server.go	10.00% <10.00%> (ø)
x/auth/client/cli/tx.go	50.00% <50.00%> (ø)
x/auth/types/codec.go	100.00% <100.00%> (ø)
x/auth/types/msgs.go	100.00% <100.00%> (ø)
x/evidence/types/evidence.go	78.94% <0.00%> (-8.56%)	⬇️
x/staking/types/validator.go	57.39% <0.00%> (-5.50%)	⬇️
x/ibc/core/04-channel/types/channel.go	83.33% <0.00%> (-0.50%)	⬇️
x/ibc/applications/transfer/types/trace.go	80.85% <0.00%> (-0.04%)	⬇️
x/gov/types/content.go	100.00% <0.00%> (ø)
... and 84 more