This method is called by the REST input parsers to allow a field
type to pre process the given $incomingValueHash before it is handled by
{@link eZ\Publish\SPI\FieldType\FieldType::fromHash()}. The
$incomingValueHash can be expected to conform to the rules that need to
apply to hashes accepted by fromHash(). The return value of this method
replaces the $incomingValueHash.