4-seasons: exit cleanly when all frames fail validation

Accessing ${images[-1]} on an empty array under set -u causes a bash
error.  Guard with an explicit check so a fully-corrupt image directory
exits with a clear message instead of a cryptic crash.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

4-seasons.sh

@@ -123,6 +123,11 @@ done
 [ "$bad"  -gt 0 ] && echo "WARNING: skipped $bad corrupt/empty frame(s) of ${#all_images[@]}"
 [ "$grey" -gt 0 ] && echo "INFO: skipped $grey grey/uniform frame(s) (SEASONS_GREY_STDDEV_MIN=${GREY_MIN})"
 
+if [ "${#images[@]}" -eq 0 ]; then
+    echo "ERROR: no valid images remain after filtering — skipping $yesterday."
+    exit 0
+fi
+
 for img in "${images[@]}"; do
     printf "file '%s'\nduration %s\n" "$img" "$INTERVAL"
 done > "$temp_file"