fix: split execution path to avoid breaking changes

[antoinegelloz]

• POST /transactions only containing postings with zero amount should be inserted without error
• POST /transactions with script only containing sends with zero amount should return "transaction has no postings" error

[codecov]

Codecov Report

Merging #427 (550e2fa) into main (871d69e) will decrease coverage by 0.39%.
The diff coverage is 71.63%.

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
- Coverage   69.38%   69.00%   -0.39%     
==========================================
  Files          95       95              
  Lines        6522     6513       -9     
==========================================
- Hits         4525     4494      -31     
- Misses       1613     1632      +19     
- Partials      384      387       +3     

Impacted Files	Coverage Δ
pkg/core/asset.go	100.00% <ø> (ø)
pkg/ledger/execute_script.go	67.55% <67.55%> (ø)
pkg/ledger/execute_txsdata.go	75.96% <75.96%> (ø)
pkg/api/controllers/script_controller.go	87.50% <100.00%> (-0.31%)	⬇️
pkg/api/controllers/transaction_controller.go	70.83% <100.00%> (-0.19%)	⬇️
pkg/ledger/ledger.go	70.17% <100.00%> (-0.26%)	⬇️
pkg/analytics/segment.go	54.07% <0.00%> (-3.71%)	⬇️

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