Element: Fix Fragment render error on empty children

[aduth]

Extracted from #6467

This pull request seeks to resolve an error which will be thrown by wp.element.renderToString when attempting to serialize a wp.element.Fragment with children absent.

Testing instructions:

Verify unit tests pass:

npm run test-unit

[jorgefilipecosta]

This PR looks good to me 👍
For testing purposes, I changed the paragraph save return to:

		return (
			<Fragment>
				<RichText.Content
					tagName="p"
					style={ styles }
					className={ className ? className : undefined }
					value={ content }
				/>
				<Fragment>
				</Fragment>
				<Fragment>
					Ok
				</Fragment>
			</Fragment>
		);

In this branch, the output is the expected in master it isn't.