refactor(loaders): make data always possibly undefined

[posva]

BREAKING CHANGE: In practice, wrong usage of the data loaders can lead to undefined data even when lazy is false

Close #319

With the introduction of errors, data can actually be undefined in many cases. This is overall more correct and produce less complicated types

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.65%. Comparing base (1dcd362) to head (537111e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
- Coverage   67.67%   67.65%   -0.03%     
==========================================
  Files          32       32              
  Lines        3072     3070       -2     
  Branches      600      600              
==========================================
- Hits         2079     2077       -2     
  Misses        987      987              
  Partials        6        6              

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