bug: notEmptyOrNull extension returns true for null values

[MadalyImbate]

Bug Description

The notEmptyOrNull extension method on SimpleValidationBuilder<String?> contains incorrect logic in its validation predicate.

Currently, the predicate is written as:

(value, entity) => value == null || value.isNotEmpty

This causes null values to evaluate to true (valid), bypassing the validation entirely when a field is null. As a result, nullable strings with null values pass the notEmptyOrNull() check instead of failing as intended.

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Additional Findings & Redundancy

While reviewing the extension file, we also noticed that NotEmptyNullableValidation.notEmpty() and NotEmptyOrNullableValidation.notEmptyOrNull() both target SimpleValidationBuilder<String?> and perform the exact same validation check (value != null && value.isNotEmpty).

Consider consolidating or deprecating one of them to avoid duplicate API behavior on nullable strings.

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Expected Behavior

notEmptyOrNull() should fail (return false) when value is null or when value is an empty string (""). It should only pass (return true) when value is non-null and non-empty.

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Steps to Reproduce

1. Define a nullable string property (e.g., String? name = null;).
2. Add a rule using builder.ruleFor((p) => p.name).notEmptyOrNull();.
3. Validate the entity.
4. Result: The validation passes as valid even though name is null.

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Suggested Fix

Update the predicate in NotEmptyOrNullableValidation:

extension NotEmptyOrNullableValidation on SimpleValidationBuilder<String?> {
  SimpleValidationBuilder<String?> notEmptyOrNull({
    String? message,
    String? code,
  }) {
    return useValidation(
       // here is the fix
      (value, entity) => value != null && value.isNotEmpty,
      code: code ?? Language.code.notEmpty,
      message: message,
    );
  }
}