Fixed string substitution for Plural Value Object when casting to string

`\Ibexa\Contracts\Core\Repository\Values\Translation\Plural` had an incorrect casting to string implementation which resulted in the effect opposite to the desired one

src/contracts/Repository/Values/Translation/Plural.php

@@ -74,7 +74,9 @@ public function __construct($singular, $plural, array $values)
      */
     public function __toString()
     {
-        return strtr(current($this->values) == 1 ? $this->plural : $this->singular, $this->values);
+        $firstValue = !empty($this->values) ? current(array_values($this->values)) : null;
+
+        return strtr((int)$firstValue === 1 ? $this->singular : $this->plural, $this->values);
     }
 }