[Bug] Linear gradient for axis lines and split lines not shown when renderer type is SVG

### Version

6.0.0

### Link to Minimal Reproduction

https://codesandbox.io/p/sandbox/m6p4p5

### Steps to Reproduce

1. Create chart with svg renderer (used bar chart in example but checked line chart as well)
2. Add axisLines and axis splitLines with linear gradient color
3. View chart, look for axes and split lines

### Current Behavior

Axis lines and split lines are not shown. Inspecting the DOM shows the path elements are there with the stroke for the linear gradient, but they do not show in the chart.

### Expected Behavior

Axis lines and split lines with a linear gradient color display in svg charts with the correct gradient.

### Environment

```markdown
- OS:
- Browser: Chrome, Firefox
- Framework:
```

### Any additional comments?

Tried passing a gradient configuration object directly and using new echarts.graphic.LinearGradient() function. Neither worked for me. If switching renderer to canvas with no other changes, the lines display with the gradient.

Saw this issue https://github.com/apache/echarts/issues/17029 which seemed similar, but for only the chart background color.