Bar overlap when barThickness = flex and data array of 1 element

### Expected behavior

Bars should not overlap.

### Current behavior

Bars overlap.

### Reproducible sample

https://codepen.io/yangfl/pen/qBVKoyY

### Optional extra steps/info to reproduce

_No response_

### Possible solution

_No response_

### Context

_No response_

### chart.js version

v3.7.1

### Browser name and version

_No response_

### Link to your project

_No response_

Start with the linked CodePen reproducible sample and compare its single-element bar configuration with the Chart.js bar layout behavior in the v3.7.1 context. Trace the relevant bar-sizing and positioning entry point, then verify that a one-element data array with barThickness set to flex renders without overlap.

Written by the indexing model from the issue text.