fix: #1350 valueTransformer transform values to null

[BrottarBasse]

Fix for issue 1350 with some changes to tests. This fix makes it so that valueTransformer can transform values to null.

Connection with issue(s)

Close #1350

Connected to #???

Solution description

Changes to implementation of value transformer so that it is still used if value is transformed to null.
I also found that the tests also had flipped values in the expect functions (actual and matcher was wrong way around)

Screenshots or Videos

To Do

• [ x] Read contributing guide
• [ x] Check the original issue to confirm it is fully satisfied
• [ x] Add solution description to help guide reviewers
• [ x] Add unit test to verify new or fixed behaviour
• [ x] If apply, add documentation to code properties and package readme

[deandreamatias]

HI @BrottarBasse please resolve pipeline errors (format form_builder.dart) to could be merge

[BrottarBasse]

HI @BrottarBasse please resolve pipeline errors (format form_builder.dart) to could be merge

Since my fix contains changes to form_builder_field.dart the checks fail.

[deandreamatias]

HI @BrottarBasse please resolve pipeline errors (format form_builder.dart) to could be merge

Since my fix contains changes to form_builder_field.dart the checks fail.

Use the command dart format . to format all files and solve the error

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.82%. Comparing base (6ed0144) to head (d6fa69a).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/form_builder_field.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1423      +/-   ##
==========================================
- Coverage   87.90%   87.82%   -0.09%     
==========================================
  Files          21       21              
  Lines         769      772       +3     
==========================================
+ Hits          676      678       +2     
- Misses         93       94       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.