fix: add missing nil checks (backport #1299)

[mergify]

Description

There are multiple functions in the codebase missing nil checks and this PR fixed them.

This pr cherry-pick the following upstream prs

• fix: types: ensure .Amount is non-nil in Coin.Validate() cosmos/cosmos-sdk#15691
• fix: prevent nil DecCoin creation when converting Coins to DecCoins cosmos/cosmos-sdk#12903

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 #1299 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a7a39e5 has failed:

On branch mergify/bp/release/v0.48.x/pr-1299
Your branch is up to date with 'origin/release/v0.48.x'.

You are currently cherry-picking commit a7a39e570.
  (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:   types/coin.go
	modified:   types/coin_test.go
	modified:   types/dec_coin.go
	modified:   types/dec_coin_test.go
	new file:   types/fuzz_test.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 66.66667% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 69.78%. Comparing base (c5fbc74) to head (98d7a10).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release/v0.48.x    #1308      +/-   ##
===================================================
- Coverage            69.79%   69.78%   -0.01%     
===================================================
  Files                  646      646              
  Lines                67526    67532       +6     
===================================================
+ Hits                 47127    47129       +2     
- Misses               18213    18216       +3     
- Partials              2186     2187       +1     

Files	Coverage Δ
types/coin.go	94.62% <100.00%> (+0.03%)	⬆️
types/dec_coin.go	77.45% <50.00%> (-0.63%)	⬇️

... and 1 file with indirect coverage changes

[zemyblue]

There is CI failure. Please check it.

[zemyblue]

There is CI failure. Please check it.

Oh, coverage fail.
I think we can skip it in this case.