Fix for the link import problem under SQLA

[szoupanos]

Wrong check of ids was fixed & also added a special case for the RETURN links that have now a different behaviour than the rest of the links.

It fixes issue #1710

[codecov-io]

Codecov Report

Merging #1769 into release_v0.12.2 will decrease coverage by <.01%.
The diff coverage is 44.44%.

@@                 Coverage Diff                 @@
##           release_v0.12.2    #1769      +/-   ##
===================================================
- Coverage            54.71%   54.71%   -0.01%     
===================================================
  Files                  246      246              
  Lines                32433    32438       +5     
===================================================
+ Hits                 17745    17747       +2     
- Misses               14688    14691       +3

Impacted Files	Coverage Δ
aiida/orm/importexport.py	78.85% <44.44%> (-0.2%)	⬇️

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