Fix post-install script failing on MacOS with unbound variable error

Fixes #90

The post-install script was failing when DDEV_UPSTREAM_PROVIDER was not
defined in the .env file. The script uses 'set -u' flag (via pipefail)
which treats unset variables as errors.

Changed the conditional check from '$DDEV_UPSTREAM_PROVIDER' to
'${DDEV_UPSTREAM_PROVIDER:-}' to use parameter expansion that provides
an empty string if the variable is unset, preventing the script from
failing when the variable doesn't exist.

Author: claude

install.yaml

@@ -135,7 +135,7 @@ post_install_actions:
   - |
     if [ -f .env ]; then
       source ".env"
-      if [[ $DDEV_UPSTREAM_PROVIDER ]]; then
+      if [[ ${DDEV_UPSTREAM_PROVIDER:-} ]]; then
         ddev dotenv set .ddev/.env.anner --ddev-upstream-provider $DDEV_UPSTREAM_PROVIDER
         git add .env.anner
         # Remove our old .env file