This enhances nodes which embed defaultNode (which is most of them),
allowing those implementations to omit a GetAttr() method. Without this,
when the kernel calls GetAttr() for an open file, FUSE will return a
size of zero, causing a read() of the file to always return 0, even
though fuse returns a nonzero number of bytes.

One can leverage this behavior to create a Linux procfs-like file, which
stat shows to be zero bytes, but can be successfully read.

N.B. This behavior is only useful if the FileSystemConnector Options
specify AttrTimeout of zero. Otherwise, the result of the first call to
GetAttr() (file == nil) is cached, and the kernel allows zero bytes to
be read. With this caching disabled, the next GetAttr() call (file !=
nil) will be used to control the number of readable bytes.