Guard FORCE_NODE_VERSION in the version error path (#7983)

Installing on the wrong node version dies with "FORCE_NODE_VERSION: unbound
variable" instead of explaining itself. The script runs under set -u and
this is the one place the variable is read bare; the three other reads all
default it.

Author: denusklo

ci/build/npm-postinstall.sh

@@ -67,7 +67,7 @@ main() {
 
   if [ "$major_node_version" -ne "${FORCE_NODE_VERSION:-24}" ]; then
     echo "ERROR: code-server currently requires node v24."
-    if [ -n "$FORCE_NODE_VERSION" ]; then
+    if [ -n "${FORCE_NODE_VERSION:-}" ]; then
       echo "However, you have overrided the version check to use v$FORCE_NODE_VERSION."
     fi
     echo "We have detected that you are on node v$major_node_version"