fix: Create GitHub siblings if enabled

[effigies]

ds006943 did not have its GitHub sibling created, and 3ea1777 looks to be the cause. I think the not should have been removed, but to fix this I just moved the check to an early return. That branch seems worth keeping in place, as we might want to do something else in the future, such as emit a warning or a logging event.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.02%. Comparing base (4d41cf4) to head (a78f0f5).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
services/datalad/datalad_service/common/github.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3663      +/-   ##
==========================================
+ Coverage   43.00%   45.02%   +2.02%     
==========================================
  Files         636      636              
  Lines       33911    33921      +10     
  Branches     1516     1595      +79     
==========================================
+ Hits        14582    15273     +691     
+ Misses      19190    18519     -671     
+ Partials      139      129      -10     

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