Error when using a widget that doesn't generate content

## Prerequisites

- [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
- [x] The issue still exists against the latest stable version of Elementor.

## Description

If the page uses a widget that returns empty content (for example, a currency switch that returns content only on store pages), an error occurs:
https://www.screencast.com/t/arI3lxD7P
https://www.screencast.com/t/HlkHatqp8avk

## Steps to reproduce

1. Add any widget that returns empty content (for example, a currency switch that returns content only on store pages) to the page
2. Save the page and update browser (press F5 or Ctrl+F5)
3. An error occurs:
https://www.screencast.com/t/arI3lxD7P
https://www.screencast.com/t/HlkHatqp8avk

## Isolating the problem

- [x] This bug happens with only Elementor plugin active (and Elementor Pro).
- [x] This bug happens with a default WordPress theme active.
- [x] I can reproduce this bug consistently using the steps above.

## Environment

System Info
```

https://pastebin.com/dxLdgAYN
```