fix(schematics): warn when ng add setup runs with no features selected

Pressing Enter without toggling any checkbox at the features prompt
silently exited with the CLI's stock "Nothing to be done." message,
giving no indication that Space selects a feature. Print a warning
explaining the checkbox controls and inviting a retry.

Author: armando-navarro

src/schematics/setup/index.ts

@@ -62,7 +62,14 @@ export const ngAddSetupProject = (
 
   const features = await featuresPrompt();
 
-  if (features.length > 0) {
+  if (features.length === 0) {
+    context.logger.warn(
+      'No features were selected, so there is nothing to set up. ' +
+      'At the "What features would you like to setup?" prompt, use the arrow keys to move, ' +
+      'press Space to select each feature you want, then Enter to confirm. ' +
+      'Re-run ng add @angular/fire to try again.'
+    );
+  } else {
 
     const firebaseTools = await getFirebaseTools();