Fix nullable return in getUserIdentifier() when username is not set

[Dylvn]

Fixes oroinc/customer-portal#14

Problem

When importing CustomerUser records without an ID (creation) and without an email address, a TypeError is triggered during Symfony validation.

The validator calls __toString() on the freshly instantiated entity, which internally calls getUserIdentifier():

public function __toString()
{
    return (string)$this->getUserIdentifier();
}

The (string) cast never executes. The TypeError is raised inside getUserIdentifier() itself, before the cast can apply.

Root cause

$username is declared as ?string at the Doctrine level, which is a valid state during entity construction (e.g. import processing before the field is hydrated). However, Symfony's UserInterface contract enforces a strict string return type on getUserIdentifier().

Fix

Return an empty string as fallback to bridge the gap between the nullable Doctrine mapping and the strict Symfony interface contract.

[mbessolov]

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