[8.x Compatibility] Doctrine_Validator::validateLength - Deprecation

[iricketson]

If a model definition contains a column of type decimal that allows nullable values, Doctrine_Table attempts to validate the length of the null value, which calls Doctrine_Validator::validateLength and ultimately throws a deprecation notice when it tries to call abs() on the null value.

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

This PR simply returns true early if the $value is null, which is what it would return anyway.

[alquerci]

The patch looks good.

I invite you to add a test case for that.
You can find an example on tests/Ticket/1385TestCase.php.

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To execute tests on your computer, you need this patch.

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