fix: no intermediary swap amount for single-legs

[dandanlen]

Pull Request

Closes: PRO-1850

Cherry-picked from release/1.7

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 72%. Comparing base (f35c07f) to head (e014543).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
state-chain/runtime/src/lib.rs	0%	0 Missing and 9 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5438    +/-   ##
======================================
- Coverage     72%     72%    -0%     
======================================
  Files        490     490            
  Lines      86916   86793   -123     
  Branches   86916   86793   -123     
======================================
- Hits       62395   62216   -179     
- Misses     21587   21641    +54     
- Partials    2934    2936     +2     

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[kylezs]

Think the error is related to the fix that will come here: #5436