Display b64 fields if their content is zero bytes

[tolikzinovyev]

Summary

For consistency with algod, display b64 fields if their content is zero bytes.

Test Plan

Ran account validation tool on one problematic account (7JBQRCAODWUE2KPTOMPQGUTQDCVJVFFO2HSSZG7OA5ETV5KNGF52YO3UOY; testnet).

[codecov-commenter]

Codecov Report

Merging #785 (e1e7641) into develop (0eee086) will decrease coverage by 0.08%.
The diff coverage is 85.18%.

@@             Coverage Diff             @@
##           develop     #785      +/-   ##
===========================================
- Coverage    54.46%   54.38%   -0.09%     
===========================================
  Files           28       28              
  Lines         3909     3915       +6     
===========================================
  Hits          2129     2129              
- Misses        1496     1501       +5     
- Partials       284      285       +1     

Impacted Files	Coverage Δ
api/handlers.go	23.96% <0.00%> (ø)
api/converter_utils.go	87.04% <100.00%> (ø)
api/pointer_utils.go	84.09% <100.00%> (+1.16%)	⬆️
idb/postgres/postgres.go	47.26% <100.00%> (-0.33%)	⬇️

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