refactor: Add type-hinting to Node-API

[CarlSchwan]

Summary

Slight API changes:

• FileInfo::getId and Node::getId now both returns an int with -1 being an invalid fileid. Previously FileInfo::getId did return null
• FileInfo::getChecksum was documented as returning a string but would from time to time returns null. Now it only return a string or an empty string. The places were we were checking for returning null or empty string, are now only checking for empty string
• LazyFolder now doesn't have any method related to IRootFolder, these are now exclusively in LazyRoot
• FileInfo requires a valid path, IMountPoint and IStorage in it's constructor. This was already the case before but not enforced and many unit tests were just passing null. There was also apps/dav/lib/Connector/Sabre/Directory.php which was a valid usecase for passing a null mountpoint and storage, which is now using a DummyMountPoint (new) and a FailedStorage

TODO

• refactor: Add return types to S3PreviewFile files_versions_s3#138

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)