XMLType on nullable columns return true instead of null

[HansPeterOrding]

The method convertToPHPValue of class XMLType returns boolean true if nullable column equals null. The use of short comparison operator is causing this issue.
Solution:

This block:

public function convertToPHPValue($value, AbstractPlatform $platform)
{
    return $value === null ?: new \SimpleXMLElement($value);
}

Should be replaced by this:

public function convertToPHPValue($value, AbstractPlatform $platform)
{
    return $value === null ? null : new \SimpleXMLElement($value);
}

Or for PHP versions >= 7:

public function convertToPHPValue($value, AbstractPlatform $platform)
{
    return $value === null ?? new \SimpleXMLElement($value);
}

See [PR-22]](#22) for PHP version > 5 fix.