fix: improve when show dropdown disabled hint

[deandreamatias]

Connection with issue(s)

Close #1242

Solution description

Only show dropdown vale when has value on items

Screenshots or Videos

Screen.Recording.2023-05-18.at.16.03.49.mov

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 #1255 (649ea3f) into main (ab3b8f7) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
+ Coverage   85.14%   85.24%   +0.09%     
==========================================
  Files          20       20              
  Lines         707      705       -2     
==========================================
- Hits          602      601       -1     
+ Misses        105      104       -1     

Impacted Files	Coverage Δ
lib/src/fields/form_builder_dropdown.dart	77.77% <100.00%> (+2.77%)	⬆️

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