Validation::_validation_match_field() - empty field gives positive match

[quetzyg]

When validating a field to see if it matches a previous one, a false positive match is returned if the later is empty.

Steps to reproduce:

• Create the validation object and add a match_field rule:
  $val = Validation::forge();
  $val->add('password_confirm', 'Password (confirm)')->add_rule('match_field', 'password');
• Fill the first field (password) with any string besides an empty one
• Leave the password_confirm field empty
• Run the validation:
  $val->run()
• It will return: true

[jschreuder]

First thoughts as given on IRC: contrary to other validation rules, match_field should not be ignored when empty.

[WanWizard]

Unless the field that is matched again is empty too. I'll fix it.