fix: fix handling of transaction metadata versioning in AssembledTransaction class.

[overcat]

Ensure compatibility with both V3 and V4 by checking for null values before accessing Soroban metadata.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.78%. Comparing base (0f7932b) to head (8f78110).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...org/stellar/sdk/contract/AssembledTransaction.java	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #723      +/-   ##
============================================
- Coverage     80.81%   80.78%   -0.04%     
  Complexity     1305     1305              
============================================
  Files           214      214              
  Lines          5071     5073       +2     
  Branches        432      433       +1     
============================================
  Hits           4098     4098              
- Misses          714      715       +1     
- Partials        259      260       +1     

Files with missing lines	Coverage Δ
...org/stellar/sdk/contract/AssembledTransaction.java	41.37% <33.33%> (-0.49%)	⬇️

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