fix: update param validation to fail on nil dec

[alexanderbez]

Description

Closes: #13550

Ensure all parameter validation ensures that decimal types are non-nil.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #13553 (f358d21) into main (d01aa5b) will decrease coverage by 0.21%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13553      +/-   ##
==========================================
- Coverage   54.08%   53.86%   -0.22%     
==========================================
  Files         652      645       -7     
  Lines       56226    55863     -363     
==========================================
- Hits        30409    30092     -317     
+ Misses      23374    23348      -26     
+ Partials     2443     2423      -20     

Impacted Files	Coverage Δ
x/mint/types/params.go	7.03% <0.00%> (-0.73%)	⬇️
x/slashing/types/params.go	0.00% <0.00%> (ø)
x/staking/types/params.go	57.62% <0.00%> (-1.51%)	⬇️
crypto/keys/internal/ecdsa/privkey.go	81.13% <0.00%> (-1.89%)	⬇️
x/staking/simulation/operations.go	74.54% <0.00%> (-1.38%)	⬇️
x/group/keeper/keeper.go	56.25% <0.00%> (-0.40%)	⬇️
tx/textual/valuerenderer/dec.go
tx/textual/valuerenderer/bytes.go
tx/textual/valuerenderer/duration.go
tx/textual/valuerenderer/coins.go
... and 3 more

[julienrbrt]

lgtm