new: [apache] complain if no first exposed port was found

apache/lib/common

@@ -555,6 +555,10 @@ __vhost_proxy_statement() {
             docker pull "$base_image"
         fi
         first_exposed_port=$(image_exposed_ports_0 "$base_image" | tr '\0' '\n' | head -n 1 | cut -f 1 -d /) || return 1
+        if [ -z "$first_exposed_port" ]; then
+            err "Failed to get first exposed port of image '$base_image'."
+            return 1
+        fi
         TARGET=$MASTER_BASE_SERVICE_NAME:$first_exposed_port
         info "No target was specified, introspection found: $TARGET"
     fi