Fix Form Validation issue with text inputs which have array as name

According to the user guide, **text** `input`s could have `array`s as name.

```HTML
<input type="text" name="arrayName[]" value="">
<input type="text" name="arrayName[]" value="">
<input type="text" name="arrayName[]" value="">
```
If when the form is submitted these inputs were blank, the `$postdata` would be an empty string (unlike `checkbox` type).
So, rules that need a non-string such as: `numeric`, would return `FALSE`, whether `required` rule is added or not.

Signed-off-by: Hashem Qolami <hashem@qolami.com>