imap: Notice undefined property in Attachment::getEmbeddedMessage()

I don’t know how to reproduce this, we just received a crash report.

On this line: https://github.com/ddeboer/imap/blob/208a29fd4bb2c3240a10ebfb51cc84f5a16dad51/src/Message/Attachment.php#L61

We’ve got a notice:

Notice: Undefined property: stdClass::$parts

I think it might be because the getStructure() method only calls AbstractPart::lazyLoadStructure() and not AbstractPart::lazyParseStructure(), but I don’t see into the code enough to know for sure.