Set empty field to blank string before validation

Addresses bug where validation fails on non-required fields
that are not present in the data. By initializing missing
fields to an empty string, validation rules like min, max,
size, etc. can be properly applied without triggering an
error due to an undefined index.

Fixes #124

src/Validator.php

@@ -124,6 +124,8 @@ protected function checkPasses(mixed $validator, string $field, ?string $ruleNam
     {
         if(!isset($this->data[$field]) && $ruleName !== 'required') return ;
 
+        $this->data[$field] = isset($this->data[$field]) ? $this->data[$field] : '';
+
         if (!$validator->passes($field, $this->data[$field], $this->data)) {
 
             $assert = isset($ruleName) && isset($this->messages[$field][$ruleName]);