feat(command): add show empty validation

app/src/main/kotlin/command/commands/Show.kt

@@ -12,8 +12,12 @@ class Show(
     override val description = "show collection elements"
 
     override fun execute() {
+        if (collectionManager.getCollection().isNotEmpty()) {
             for (product in collectionManager.getCollection()) {
                 io.println(product.toString())
             }
+        } else {
+            io.println("коллекция пустая")
+        }
     }
 }