Handle empty layers in dmverity-vhd

[DomAyre]

We see a bug where getting hashes for an image which includes a layer that does nothing (such as chmod-ing files which already have those permissions) causes a layer hash which is a blank string.

The root cause is that when checking for layers in the image, we treat TarReader.Next() returning an io.EOF as not a tar file rather than a valid but empty tar file, therefore we don't attempt to generate a hash for it

[helsaawy]

For the empty layer case, is the layer a 0-byte tar file, or a tar file with empty headers?

[anmaxvl]

@DomAyre, we moved out the binary to a separate repo: https://github.com/microsoft/integrity-vhd. we'll be removing dmverity-vhd from this repo later.