fix: ignore backspace when the stack is empty

samples/calc-02/index.js

@@ -162,9 +162,10 @@ function execute(name) {
 		} else if (name === 'backspace') {
 			if (calc.isEditing) {
 				inputToX('Backspace');
-			} else {
+			} else if (calc.isValidIndex(0)) {
 				calc.remove(0);
 			}
+
 		} else if (name === 'swap') {
 			pushEditingValueIfNeeded();
 			if (calc.isValidIndex(1)) {