Added null protection on validation

[rcerljenko]

We use this package in our company and we had a situation when frontend sends a null value to the validated field causing an TypeError exception to be thrown like so:

/.../vendor/sunspikes/clamav-validator/src/ClamavValidator/ClamavValidator.php:140

Sunspikes\ClamavValidator\ClamavValidator::getFilePath(): Return value must be of type string, null returned

This PR resolves that by skipping a validation if the validated field is null.

[rcerljenko]

@sunspikes could you take a look please.. thanks!

[sunspikes]

@rcerljenko sorry that i missed this PR, If you just want the field to accept null values, did you try: https://laravel.com/docs/9.x/validation#rule-nullable

You could basically do

$rules = [
    'file' => 'nullable|clamav',
];

[marcovo-peercode]

Adding nullable could be a work-around, but not a solution to this problem. In line with standard Laravel rules, a validation rule should always return a valid conclusion (true/false) rather than throwing an exception.

In particular: I think the best approach, in line with standard Laravel rules, would be to let the rule return false whenever an empty value is received. This in contrast with the here proposed change.