fix(state sync): fix fallback logic

[saketh-are]

Incorrect logic was causing state sync to attempt using the fallback source even if it's None.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.61%. Comparing base (ba6c707) to head (f589ecc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12254      +/-   ##
==========================================
- Coverage   71.62%   71.61%   -0.02%     
==========================================
  Files         837      837              
  Lines      167105   167105              
  Branches   167105   167105              
==========================================
- Hits       119696   119677      -19     
- Misses      42180    42208      +28     
+ Partials     5229     5220       -9     

Flag	Coverage Δ
backward-compatibility	0.16% <0.00%> (ø)
db-migration	0.16% <0.00%> (ø)
genesis-check	1.25% <0.00%> (ø)
integration-tests	38.88% <100.00%> (+<0.01%)	⬆️
linux	71.22% <100.00%> (-0.02%)	⬇️
linux-nightly	71.20% <100.00%> (-0.01%)	⬇️
macos	53.70% <0.00%> (-0.61%)	⬇️
pytests	1.57% <0.00%> (ø)
sanity-checks	1.37% <0.00%> (ø)
unittests	65.42% <0.00%> (-0.02%)	⬇️
upgradability	0.21% <0.00%> (ø)

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