fix(dstack-ingress): stop the placeholder path reporting itself as a failure

On a first tls-alpn-01 run the log read:

    Generating placeholder certificate for app.example.com
    Warning: Cert files missing for app.example.com, skipping

The warning is what build_combined_pems prints whenever lego has no
certificate yet, which on the first pass is exactly the situation the
placeholder exists to cover. Nothing is wrong -- skipping is what keeps
the placeholder in place so haproxy can bind and forward acme-tls/1 --
but back to back with the line above it reads like the placeholder was
discarded, and it is the first thing a new deployment shows.

Distinguish the two cases: keeping a placeholder is normal, having
neither a certificate nor a placeholder is not. dns-01 has no
placeholder, so its copy keeps the original wording.

Author: kvinwang

custom-domain/dstack-ingress/scripts/tlsalpn.sh

@@ -161,8 +161,13 @@ build_combined_pems() {
             cat "$fullchain" "$privkey" > "$combined"
             chmod 600 "$combined"
             echo "Combined PEM created: ${combined}"
+        elif [ -f "$combined" ]; then
+            # Expected on a first run: ensure_placeholder_certs has already put a
+            # self-signed certificate here so haproxy can bind and forward
+            # acme-tls/1, and the real one does not exist yet. Leave it alone.
+            echo "No certificate for ${domain} yet; serving the placeholder"
         else
-            echo "Warning: Cert files missing for ${domain}, skipping"
+            echo "Warning: no certificate or placeholder for ${domain}, skipping"
         fi
     done <<<"$(get-all-domains.sh)"
 }