Accept upstreamUri as string for proxy app install

src/reverseproxy.js

@@ -510,6 +510,10 @@ async function writeAppNginxConfig(app, fqdn, type, bundle) {
 
         if (app.manifest.id === constants.PROXY_APP_APPSTORE_ID) {
             data.endpoint = 'external';
+
+            // prevent generating invalid nginx configs
+            if (!app.upstreamUri) throw new BoxError(BoxError.BAD_FIELD, 'upstreamUri cannot be empty');
+
             data.upstreamUri = app.upstreamUri;
         }