[Bug]: avatar should render an alt tag even if no label is specified to avoid a11y violations.

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

avatar

### Expected behavior

```html

```

### Actual behavior

```html

```

### Screenshots

_No response_

### What browsers are you seeing the problem in?

Microsoft Edge, Safari, Chrome, Firefox

### How can we reproduce this issue?

1. Go to https://opensource.adobe.com/spectrum-web-components/components/avatar/#sizes
2. Remove label property
3. Open Dev Console
4. Check img doesnt have alt attribute

### Sample code or abstract reproduction which illustrates the problem

_No response_

### Severity

None

### Logs taken while reproducing problem

_No response_

### Would you like to track this issue in Jira?

- [x] Yes, please tell me the ticket number!