Prevent "Background"-Folder creation for guest users

[SystemKeeper]

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Steps to reproduce

1. Be a guest user (from the guests app)
2. Open the media settings in talk

While the upload button is hidden, we still try to create the Backgrounds folder:

(not totally sure why the Talk/ part is missing, I've seen the error for the correct path for guest users)

spreed/src/components/MediaSettings/VideoBackgroundEditor.vue

Lines 178 to 186 in f44dc72

	try {
	// Create the backgrounds folder if it doesn't exist
	const client = getDavClient()
	if (await client.exists(absoluteBackgroundsFolderPath) === false) {
	await client.createDirectory(absoluteBackgroundsFolderPath)
	}
	} catch (error) {
	console.debug(error)
	}

If we know we cannot upload anything, we should not try to create the folder.