[Validator] Fix NotBlank validator to treat whitespace-only strings as blank

[lionelkimbs]

Problem:
The NotBlank validator previously failed to recognize strings consisting solely of whitespace characters (e.g., " " or " ") as blank.

Solution:
This PR updates the NotBlank validator's logic to treat strings that contain only whitespace characters as blank.

Key Changes:
Updated the validation logic in the NotBlankValidator class to trim value if type is string, then proceed to current checking.

Testing:
Comprehensive unit tests have been added to verify the correct behavior of the NotBlank validator with whitespace-only strings. All existing tests have been run to ensure that this change does not introduce regressions in other parts of the application.