LibJS: Optimize & Bring String.prototype.repeat closer to the spec

Specifically, we immediately return an empty string when `this` is an
empty string, instead of wasting time in a loop doing nothing N times.

Userland/Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -167,21 +167,30 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::repeat)
  auto string = ak_string_from(vm, global_object);
  if (!string.has_value())
  return {};
- if (!vm.argument_count())
- return js_string(vm, String::empty());
- auto count = vm.argument(0).to_integer_or_infinity(global_object);
+
+ auto n = vm.argument(0).to_integer_or_infinity(global_object);
  if (vm.exception())
  return {};
- if (count < 0) {
+
+ if (n < 0) {
  vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "positive");
  return {};
  }
- if (Value(count).is_infinity()) {
+
+ if (Value(n).is_positive_infinity()) {
  vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "finite");
  return {};
  }
+
+ if (n == 0)
+ return js_string(vm, String::empty());
+
+ // NOTE: This is an optimization, it is not required by the specification but it produces equivalent behaviour
+ if (string->is_empty())
+ return js_string(vm, String::empty());
+
  StringBuilder builder;
- for (size_t i = 0; i < count; ++i)
+ for (size_t i = 0; i < n; ++i)
  builder.append(*string);
  return js_string(vm, builder.to_string());
 }