fix: SEP-10, value of the first op should not be null

[overcat]

See stellar/js-stellar-sdk#592

[codecov]

Codecov Report

Merging #394 into dev will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #394      +/-   ##
==========================================
+ Coverage   99.00%   99.04%   +0.04%     
==========================================
  Files         127      127              
  Lines        7644     7660      +16     
==========================================
+ Hits         7568     7587      +19     
+ Misses         76       73       -3     

Flag	Coverage Δ
#unittests	99.04% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
stellar_sdk/sep/stellar_web_authentication.py	92.76% <100.00%> (+0.09%)	⬆️
tests/sep/test_stellar_web_authentication.py	100.00% <100.00%> (ø)
stellar_sdk/client/aiohttp_client.py	90.90% <0.00%> (+3.03%)	⬆️