Update dependencies to null-safe versions

[vitusortner]

Updates the following dependencies to their null-safe version:

• build
• build_test
• dart_style
• source_gen

#504

Closes #518
Closes #519

[codecov]

Codecov Report

Merging #520 (84692ee) into develop (b0601d8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #520   +/-   ##
========================================
  Coverage    84.34%   84.34%           
========================================
  Files           73       73           
  Lines         1846     1846           
========================================
  Hits          1557     1557           
  Misses         289      289           

Flag	Coverage Δ
floor	84.34% <100.00%> (ø)
floor_generator	83.71% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rator/lib/misc/change_method_processor_helper.dart	71.42% <ø> (ø)
.../lib/misc/extension/type_converters_extension.dart	100.00% <ø> (ø)
...rator/lib/processor/deletion_method_processor.dart	77.77% <ø> (ø)
.../lib/processor/error/database_processor_error.dart	100.00% <ø> (ø)
...or/lib/processor/error/entity_processor_error.dart	16.12% <ø> (ø)
...generator/lib/processor/error/processor_error.dart	11.11% <ø> (ø)
.../processor/error/query_method_processor_error.dart	100.00% <ø> (ø)
...tor/lib/processor/error/query_processor_error.dart	100.00% <ø> (ø)
...lib/processor/error/queryable_processor_error.dart	100.00% <ø> (ø)
...ssor/error/transaction_method_processor_error.dart	100.00% <ø> (ø)
... and 11 more

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