fix: update param validation to fail on nil dec (backport #1323)

[mergify]

Description

• fix: update param validation to fail on nil dec
  • refer: fix: update param validation to fail on nil dec cosmos/cosmos-sdk#13553

Motivation and context

How has this been tested?

Screenshots (if appropriate):

Checklist:

• I followed the contributing guidelines and code of conduct.
• I have added a relevant changelog to CHANGELOG.md
• I have added tests to cover my changes.
• I have updated the documentation accordingly.
• I have updated API documentation client/docs/swagger-ui/swagger.yaml

---

This is an automatic backport of pull request #1323 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 55b6566 has failed:

On branch mergify/bp/release/v0.48.x/pr-1323
Your branch is up to date with 'origin/release/v0.48.x'.

You are currently cherry-picking commit 55b6566bf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   x/mint/types/params.go
	modified:   x/slashing/types/params.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 69.74%. Comparing base (69a3ec7) to head (696d83e).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release/v0.48.x    #1325      +/-   ##
===================================================
- Coverage            69.76%   69.74%   -0.03%     
===================================================
  Files                  646      646              
  Lines                67573    67594      +21     
===================================================
  Hits                 47142    47142              
- Misses               18242    18263      +21     
  Partials              2189     2189              

Files	Coverage Δ
x/slashing/types/params.go	0.00% <0.00%> (ø)
x/mint/types/params.go	6.47% <0.00%> (-0.62%)	⬇️