Make floor_generator null safe internally

[vitusortner]

Part of #375.

[codecov]

Codecov Report

Merging #434 (79f5e19) into nullsafety (4502f77) will increase coverage by 0.10%.
The diff coverage is 94.82%.

@@              Coverage Diff               @@
##           nullsafety     #434      +/-   ##
==============================================
+ Coverage       82.75%   82.86%   +0.10%     
==============================================
  Files              68       68              
  Lines            1653     1663      +10     
==============================================
+ Hits             1368     1378      +10     
  Misses            285      285              

Flag	Coverage Δ
floor	82.86% <94.82%> (+0.10%)	⬆️
floor_generator	82.68% <94.82%> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
...loor_generator/lib/misc/annotation_expression.dart	100.00% <ø> (ø)
...enerator/lib/misc/change_method_writer_helper.dart	100.00% <ø> (ø)
...ator/lib/misc/extension/dart_object_extension.dart	100.00% <ø> (ø)
...or/lib/misc/extension/list_equality_extension.dart	100.00% <ø> (ø)
...tor/lib/misc/extension/set_equality_extension.dart	100.00% <ø> (ø)
...or_generator/lib/misc/extension/set_extension.dart	100.00% <ø> (ø)
...sc/extension/type_converter_element_extension.dart	100.00% <ø> (ø)
floor_generator/lib/misc/string_utils.dart	100.00% <ø> (ø)
floor_generator/lib/misc/type_utils.dart	100.00% <ø> (ø)
floor_generator/lib/processor/dao_processor.dart	100.00% <ø> (ø)
... and 49 more

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