Re: Fix detection of truncated zstd-compressed backups

Hello

An automatic post-commit claude review pointed out that this broke
restoring directory format dumps with an empty large object, when
compressed with zstd or lz4:

SELECT lo_create(0);

$ pg_dump -Fd -Z zstd -f dump postgres
$ pg_restore -f /dev/null dump
pg_restore: error: could not decompress data: compressed stream is
incomplete

$ pg_dump -Fd -Z lz4 -f dump postgres
$ pg_restore -f /dev/null dump
pg_restore: error: could not read from input file: Input/output error
pg_restore: error: could not read from input file: Input/output error

The compressed file handles only start a frame on the first write, so
the blob_NNN.dat.zst/.lz4 file of an empty large object is 0 bytes,
and the new check treats that as a truncated stream. Custom format and
gzip are fine.

The attached patch makes the reader accept a file with no input at all
as a complete stream, a stream that started a frame and then stopped
is still rejected.