fix: detect_hosts() aborts when REMOTE_ID is empty on single-server sparse checkout

${!REMOTE_ID:-unknown} with REMOTE_ID="" is a fatal bash error that terminates
the entire function — not just the line. All subsequent aliases (EMBY_CONTAINER,
TRANSCODE_SSD, RAMDISK_LOW_GB, SSH_KEY) were never set, causing awk syntax errors
and "SSD fallback path missing" warnings in every transcode script invocation.

Guard with an explicit empty check instead of relying on the :- fallback,
which bash does not apply when the indirect expansion itself is invalid.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common.sh

@@ -452,7 +452,7 @@ detect_hosts() {
 
     # ── Set host name strings ─────────────────────────────────────────────────
     LOCAL_SERVER_NAME="${!MY_ID}"
-    REMOTE_SERVER_NAME="${!REMOTE_ID:-unknown}"
+    [[ -n "$REMOTE_ID" ]] && REMOTE_SERVER_NAME="${!REMOTE_ID}" || REMOTE_SERVER_NAME="unknown"
 
     # ── Set SSH key ───────────────────────────────────────────────────────────
     local ssh_key_var="${MY_ID}_SSH_KEY"