Fixed Sabre Node implementation to correctly return timestamps as int

Replacement PR for #8098 
Fixes #7921

Negative timestamps were returned as string and were confusing other
Sabre API like Sabre_DAV_Property_GetLastModified.
This fix makes sure the timestamp is returned as int when defined.

Please review @DeepDiver1975 @icewind1991 

To test this:
1. Create a file "test.txt" in a local sync folder
2. Wait for it to be uploaded successfully
3. Run `touch -d 1960-11-14 test.txt`
4. Create another file to retrigger sync

Before the fix: error occured while checking if "test.txt" needs syncing
After the fix: no error