"touchCallback" with "FlLongPressEnd" event return not empty "lineBarSpots"

**Describe the bug**
```dart
touchCallback: (FlTouchEvent event, LineTouchResponse? touchResponse) {
print(event) // FlLongPressEnd
print(touchResponse.lineBarSpots.isEmpty) // false
},
```
**Expected behavior**
To be empty like other end events.

**Versions**
Flutter: 3.24.5
FlChart: 0.69.2

Start by reproducing the touchCallback behavior for FlLongPressEnd with the provided Flutter and fl_chart versions, then trace the line-chart touch handling reached by that callback. Done means lineBarSpots is empty for this end event, consistently with the other end events, with the relevant behavior covered by an existing or added test if the project provides one.

Written by the indexing model from the issue text.