Fix null pointer dereference, keep FileModifyJob metadata and serialize with options.

Summary:
This patch fixes a couple issues regarding FileAbstractUploadJob and FileModifyJob:
- Fix null pointer dereference that was causing [[ https://bugs.kde.org/show_bug.cgi?id=411634 | BUG 411634 ]]. This was introduced in {D22787} and in the affected block of code the mime-type must be determined by LibKGAPI as no File metadata was provided.
- Pass File metadata from FileModifyJob ctor to FileAbstractUploadJob ctor. This was letting KIO GDrive trigger the above bug when trying to specify the modified files mime-type.
- Use provided serialization options also in FilePtr serialization during building multipart request.

BUG: 411634

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #libkgapi, kde-pim

Tags: #kde_pim, #libkgapi

Differential Revision: https://phabricator.kde.org/D24301