Do not prune after a backup that found a configured path empty (closes #388)

snapshot create --prune of a snapshot whose path was an empty
directory, as the mount point of a volume that is not mounted is,
recorded an empty snapshot and then purged the older snapshots that
held the data, with their blobs.

The scan now counts the entries it finds in each configured path.
Under --prune, a run in which a path of a snapshot it made held
nothing skips the prune, keeps every older snapshot, warns naming the
path and snapshot, and exits non-zero, as
#382 does for unreadable
files. Only the current run is looked at. Without --prune nothing
changes. The README's --prune text and the flag's help say so.

Model: opus-5-5

README.md

@@ -325,7 +325,13 @@ value where a list belongs or a word where a number belongs), is refused.
   `--skip-errors` skipped a file or directory it could not read or list, the
   prune does not run and every older snapshot is kept, since one of them may
   be the only one that holds what was skipped; the command says so and exits
-  non-zero. A file removed during the backup does not stop the prune.
+  non-zero. A file removed during the backup does not stop the prune. The
+  same holds when the backup found nothing in a configured path, as in the
+  mount point of a volume that is not mounted: the prune does not run, every
+  older snapshot is kept, and the command names the path and exits non-zero,
+  on every run until the path holds something again. Take a path that is
+  empty on purpose out of the config, or remove the older snapshots by hand
+  with `snapshot purge`.
 * `--keep-newer-than <duration>`: With `--prune`, keep snapshots newer than
   this duration instead of only the latest (e.g. `4w`, `30d`, `6mo`, `1y`).
   Without `--prune`, or with a value that is not a valid duration, the