inode.c: do not create blob from zero-sized data buffer

inode.c: do not create blob from zero-sized data buffer

Moving forwards towards eliminating zero-size blobs: make
inode_add_stream_with_data() and inode_replace_stream_data() use a NULL
blob pointer for zero-length buffers.  Currently this can happen if a
Windows reparse point has no reparse data or if an empty named data
stream is created in a mounted WIM image.