Add RequiredNonEmptyValidator

[laogao]

Connection with issue(s)

Close #377

Solution description

Creates a separate RequiredNonEmptyValidator to cover cases of empty Iterables ,Maps and Sets.

Screenshots or Videos

To Do

• [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

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 :tada:

Comparison is base (711568b) 95.88% compared to head (84e0557) 95.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   95.88%   95.91%   +0.03%     
==========================================
  Files          69       70       +1     
  Lines        1338     1348      +10     
==========================================
+ Hits         1283     1293      +10     
  Misses         55       55              

Impacted Files	Coverage Δ
lib/src/validators/required_validator.dart	100.00% <ø> (ø)
...b/src/validators/required_non_empty_validator.dart	100.00% <100.00%> (ø)
lib/src/validators/validators.dart	100.00% <100.00%> (ø)

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