feat(aminojson): expose NullSliceAsEmptyEncoder and add null_slice_as_empty encoding option

[forkfury]

Description

Exposes NullSliceAsEmptyEncoder as a public function to allow apps to use it for their own fields.
Adds "null_slice_as_empty" encoding option that can be used in protobuf annotations for any slice type.

Changes:

• Export NullSliceAsEmptyEncoder function
• Register "null_slice_as_empty" field encoder (in addition to "legacy_coins")
• Update documentation with usage examples

Fixes #25222

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.66%. Comparing base (002ae6e) to head (ec42fe1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25539      +/-   ##
==========================================
+ Coverage   68.34%   70.66%   +2.32%     
==========================================
  Files         857      851       -6     
  Lines       55714    55752      +38     
==========================================
+ Hits        38075    39397    +1322     
+ Misses      17639    16355    -1284     

Files with missing lines	Coverage Δ
x/tx/signing/aminojson/encoder.go	34.75% <100.00%> (ø)
x/tx/signing/aminojson/json_marshal.go	76.59% <100.00%> (ø)

... and 66 files with indirect coverage changes

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[aljo242]

lgtm - can we get a changelog entry and some testing?

[aljo242]

@forkfury CI is failing