Make foreign key mapping null safe

[vitusortner]

No description provided.

[codecov]

Codecov Report

Merging #451 (84b6517) into nullsafety (0af0b1b) will increase coverage by 0.54%.
The diff coverage is 97.50%.

@@              Coverage Diff               @@
##           nullsafety     #451      +/-   ##
==============================================
+ Coverage       83.04%   83.59%   +0.54%     
==============================================
  Files              68       68              
  Lines            1646     1670      +24     
==============================================
+ Hits             1367     1396      +29     
+ Misses            279      274       -5     

Flag	Coverage Δ
floor	83.59% <97.50%> (+0.54%)	⬆️
floor_generator	82.98% <97.50%> (+0.62%)	⬆️

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Impacted Files	Coverage Δ
...b/misc/extension/foreign_key_action_extension.dart	85.71% <85.71%> (ø)
...ator/lib/misc/extension/dart_object_extension.dart	100.00% <100.00%> (ø)
...loor_generator/lib/processor/entity_processor.dart	80.00% <100.00%> (+0.28%)	⬆️
...or/lib/processor/error/entity_processor_error.dart	16.12% <100.00%> (+12.42%)	⬆️
...ator/lib/processor/insertion_method_processor.dart	89.18% <100.00%> (+0.95%)	⬆️
...nerator/lib/processor/update_method_processor.dart	83.33% <100.00%> (+1.51%)	⬆️
floor_generator/lib/processor/view_processor.dart	100.00% <100.00%> (+4.34%)	⬆️
floor_generator/lib/value_object/foreign_key.dart	66.66% <100.00%> (ø)
floor_generator/lib/processor/field_processor.dart	87.87% <0.00%> (+0.37%)	⬆️
... and 2 more

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