PieChartData incorrect center color for one PieChartSectionData with opacity color in sections

**Describe the bug**
The center of the pie chart has an incorrect color when it has one PieChartSectionData in the sections parameter and the PieChartSectionData color has opacity

**To Reproduce**

````
import 'package:collection/collection.dart';
import 'package:fl_chart/fl_chart.dart';
import 'package:flutter/material.dart';

void main() => runApp(const FlChartPieCharts());

class FlChartPieCharts extends StatelessWidget {
const FlChartPieCharts({super.key});

static const List pieChartData1 = [1];
static const List pieChartData2 = [0.5, 0.5];

@override
Widget build(BuildContext context) {
return const MaterialApp(
title: 'FL Chart Pie Charts',
home: Scaffold(
body: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
TestPieChart(pieChartData: pieChartData1),
SizedBox(height: 32),
TestPieChart(pieChartData: pieChartData2)
],
),
),
);
}
}

class TestPieChart extends StatefulWidget {
const TestPieChart({super.key, required this.pieChartData});

final List pieChartData;

@override
State createState() => _TestPieChartState();
}

class _TestPieChartState extends State {
int touchedIndex = -1;

@override
Widget build(BuildContext context) {
return AspectRatio(
aspectRatio: 2,
child: PieChart(
PieChartData(
centerSpaceColor: Colors.white,
pieTouchData: PieTouchData(
touchCallback: (FlTouchEvent event, pieTouchResponse) {
setState(() {
if (pieTouchResponse == null || pieTouchResponse.touchedSection == null) {
return;
}
touchedIndex = pieTouchResponse.touchedSection!.touchedSectionIndex;
});
},
),
sections: [
...widget.pieChartData.mapIndexed(
(index, data) => PieChartSectionData(
value: data,
radius: 32,
color: touchedIndex == index ? Colors.green : Colors.green.withOpacity(0.5),
),
),
],
),
),
);
}
}

````
**Screenshots**

**Versions**
- which version of the Flutter are you using?
3.22.3
- which version of the FlChart are you using? fl_chart:
0.68.0

Start by running the provided Flutter reproduction and inspect the PieChart, PieChartData, and PieChartSectionData entry points involved in rendering the center and section colors. Reproduce the single-section case with a translucent color, compare it with the two-section case, and verify that the center remains the configured white centerSpaceColor.

Written by the indexing model from the issue text.