Fix 687 and 1231

[deandreamatias]

Connection with issue(s)

Close #687
Close #1231

Solution description

• Solved assert error on empty String
• Replace widget for DropdownButtonFormField

Screenshots or Videos

Screencast.from.07-05-23.01.23.14.webm

To Do

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

[codecov]

Codecov Report

Merging #1239 (f7965e0) into main (c48671f) will decrease coverage by 0.55%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##             main    #1239      +/-   ##
==========================================
- Coverage   85.67%   85.12%   -0.55%     
==========================================
  Files          21       21              
  Lines         733      733              
==========================================
- Hits          628      624       -4     
- Misses        105      109       +4     

Impacted Files	Coverage Δ
lib/src/fields/form_builder_dropdown.dart	75.00% <64.28%> (-20.00%)	⬇️

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