[VFS] DelegateFileObject does not prevent NullPointerExceptions
while working with VirtualFileSystemProvider (which seems to be the only
user of DelegateFileObject) I noticed that if a delegated file is created
without a backing file some methods (like doCreateFolder()) do not
properly catch this condition. It produces a rather ugly NPE.
Some methods act sane on file == null, but a lot of the delegating methods