BUG #19891: pg_restore ignores an explicitly empty -F argument

The following bug has been logged on the website:

Bug reference: 19891
Logged by: Faye
Email address: imchifan(at)mail(dot)com
PostgreSQL version: 18.6
Operating system: Linux/amd64
Description:

pg_restore treats an explicitly empty -F/--format argument as though the
option were absent and proceeds with archive-format auto-detection. With
empty input, I get an input-file error, while another invalid format is
rejected during option validation.

Because -F requires an archive format and accepts only c, d, or t, an
explicitly supplied empty value should be rejected as an unrecognized
archive format rather than enabling auto-detection.

How to repeat:

Run these commands with pg_restore on PATH:

```
pg_restore -f - -F '' </dev/null
pg_restore -f - -F garbage </dev/null
```

PostgreSQL 18.6 produces:

```
pg_restore: error: input file is too short (read 0, expected 5)
pg_restore: error: unrecognized archive format "garbage"; please specify
"c", "d", or "t"
```

The empty value should instead be rejected consistently with the other
invalid value, identifying the empty string as an unrecognized archive
format.

The issue was reproduced with pg_restore from PostgreSQL 18.6 (Debian
18.6-1.pgdg13+2) on Linux/amd64. The option handler records the format name
only when the argument is nonempty, so an explicitly empty argument remains
indistinguishable from an omitted -F option. This occurs when pg_restore is
invoked with an explicitly empty archive-format argument.