ssh: Fix adding an ed25519 key with a zero length comment.

* agent/command-ssh.c (sexp_key_construct): Do not put an empty string
into an S-expression.
(stream_read_string): Do not not try to a read a zero length block.

Actually we could handles this different by not putting a comment tag
into the s-expression, however this requires more code and at other
places we already return "(none)" instead of an empty comment.

The second fix is more or less a cosmetic thing to get better error
messages in case the underlying read system call returns an error.

• GnuPG-bug-id: T5794