SVG Icon Disappears When Using buttonContent in ImageTool Configuration

When the `buttonContent` property is set in the configuration for the `ImageTool` of EditorJS, the SVG icon disappears, leaving only the text message visible.

## Examples:

### Example 1:

```javascript
import ImageTool from '@editorjs/image';

var editor = EditorJS({
...

tools: {
...
image: {
class: ImageTool,
config: {
endpoints: {
byFile: 'http://localhost:8008/uploadFile', // Your backend file uploader endpoint
byUrl: 'http://localhost:8008/fetchUrl', // Your endpoint that provides uploading by Url
}
}
}
}

...
});
```


### Example 2:

```javascript
import ImageTool from '@editorjs/image';

var editor = EditorJS({
...

tools: {
...
image: {
class: ImageTool,
config: {
endpoints: {
byFile: 'http://localhost:8008/uploadFile', // Your backend file uploader endpoint
byUrl: 'http://localhost:8008/fetchUrl', // Your endpoint that provides uploading by Url
},
buttonContent: 'Select an image',
}
}
}

...
});
```


## Temporary Solution:

A temporary fix is to include the SVG icon within the buttonContent property:

```javascript
import ImageTool from '@editorjs/image';

var editor = EditorJS({
...

tools: {
...
image: {
class: ImageTool,
config: {
endpoints: {
byFile: 'http://localhost:8008/uploadFile', // Your backend file uploader endpoint
byUrl: 'http://localhost:8008/fetchUrl', // Your endpoint that provides uploading by Url
},
buttonContent: ' Select an image',
}
}
}

...
});
```

Start at the ImageTool configuration and trace how buttonContent is rendered for the image selection button. Compare the default button with the custom-content case, then verify that setting buttonContent preserves the SVG icon alongside the supplied text.

Written by the indexing model from the issue text.