Support saving blank options for radio fields and checkbox field

[AbdiTolesa]

Fix https://github.com/Strategy11/formidable-pro/issues/3385

Since I noticed there were other proposed solutions to this like the one in #1171, I'm just trying a different approach of allowing blank options to be saved/retrieved without being trimmed.

I'm not quite sure if this is worth consider or not but it could be used to brainstorm on it.

Screencast shared by Mike on how to replicate it:

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0eb0725) 29.30% compared to head (7a05e03) 29.31%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1358   +/-   ##
=========================================
  Coverage     29.30%   29.31%           
- Complexity     7259     7261    +2     
=========================================
  Files           105      105           
  Lines         23927    23930    +3     
=========================================
+ Hits           7013     7016    +3     
  Misses        16914    16914           

Files	Coverage Δ
classes/controllers/FrmFieldsController.php	33.10% <ø> (+0.07%)	⬆️
classes/models/FrmEntryMeta.php	25.49% <100.00%> (ø)
classes/models/FrmEntryValidate.php	63.12% <100.00%> (ø)
classes/helpers/FrmAppHelper.php	38.16% <75.00%> (+0.07%)	⬆️

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[AbdiTolesa]

@Crabcyborg I have drafted this PR to know what you think about it, I'm thinking of an alternative approach to support saving blank options in radio, radio based and checkbox fields. This at least has solved the issue for me. The blank options won't be selected by default. What's your thoughts on it?