PHP 8 Deprecation - Validate length on null float/decimal

[iricketson]

If your model contains a nullable float/decimal, and a null value is passed into the Doctrine_Validator::validateLength static method, the validator attempts to call abs on a null value, resulting in a PHP Deprecation Error:

PHP message: PHP Deprecated: abs(): Passing null to parameter #1 ($num) of type int|float is deprecated in /path/to/friendsofsymfony1/doctrine1/lib/Doctrine/Validator.php on line 100"

It seems that any value with a null value should pass this validator so this PR simply returns early if the value is null.

[thePanz]

Thanks @iricketson @alquerci
Merged