Update billing schema and plan picker logic

Modified the logic in create-billing-schema.ts so if a plan contains more than one line item, it no longer returns true but false. In plan-picker.tsx, added a condition to check if a product is selected before setting the 'planId'. Also, the 'planId' is now set based on the selected plan's id rather than an empty string.

packages/billing/core/src/create-billing-schema.ts

@@ -215,13 +215,13 @@ const BillingSchema = z
         for (const product of schema.products) {
           for (const plan of product.plans) {
             if (plan.lineItems.length > 1) {
-              return true;
+              return false;
             }
           }
         }
-
-        return true;
       }
+
+      return true;
     },
     {
       message: 'Lemon Squeezy only supports one line item per plan',