🐞 BUG - Rem no value adds empty global variables

## 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
Presented in the 3.2 core beta. I gave access to @shilo-ey to a site that had an issue where global variables were added when it should default to theme when no global fonts value in Elementor site settings is changed. @shilo-ey discovered that when there is no global fonts size under REM it tries to add variables that don't have a value, but changed to PX with no value then the problem isn't present. Just creating this issue to make sure the issue @shilo-ey help troubleshoot will be fixed in version 3.2.2 when released.
Thanks 👍


https://user-images.githubusercontent.com/55829113/116008586-3d68dc00-a60d-11eb-8f0a-b1602c8390a5.mp4

[https://experiments.elementorwp.uk/wp-content/uploads/sites/10/2021/04/2021-04-21-14-26-58.mp4](https://experiments.elementorwp.uk/wp-content/uploads/sites/10/2021/04/2021-04-21-14-26-58.mp4)

## Steps to reproduce

## 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
```
== Server Environment ==
Operating System: Linux
Software: Apache
MySQL version: MariaDB Server v5.7.31
PHP Version: 7.4.16
PHP Max Input Vars: 2500
PHP Max Post Size: 128M
GD Installed: Yes
ZIP Installed: Yes
Write Permissions: All right
Elementor Library: Connected

== WordPress Environment ==
Version: 5.7.1
Site URL: https://experiments.elementorwp.uk
Home URL: https://experiments.elementorwp.uk
WP Multisite: Yes
Max Upload Size: 128 MB
Memory limit: 2048M
Permalink Structure: /%year%/%monthnum%/%day%/%postname%/
Language: en-GB
Timezone: Europe/London
Debug Mode: Inactive

== Theme ==
Name: Astra
Version: 3.3.2
Author: Brainstorm Force
Child Theme: No

== User ==
Role: administrator
WP Profile lang: en_GB
User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.72 Safari/537.36

== Active Plugins ==
Astra Pro
Version: 3.3.1
Author: Brainstorm Force

Elementor
Version: 3.2.1
Author: Elementor.com

Elementor Pro
Version: 3.2.1
Author: Elementor.com
== Network Plugins ==
Elementor
Version: 3.2.1
Author: Elementor.com

WP Mail SMTP
Version: 2.7.0
Author: WPForms
== Must-Use Plugins ==
StackCache
Version:
Author: Stack CP

== Elementor - Compatibility Tag ==

Elementor Pro: incompatible
== Elementor Pro - Compatibility Tag ==

== Elements Usage ==

wp-page : 1
button : 1
call-to-action : 3
heading : 2
lottie : 1
slides : 1
text-editor : 1
== Log ==

Log: showing 13 of 132021-03-19 09:16:15 [info] Elementor data updater process has been completed. [array (
'plugin' => 'Elementor',
'from' => '3.1.1',
'to' => '3.1.4',
)]
2021-03-19 09:16:16 [info] Elementor data updater process has been completed. [array (
'plugin' => 'Elementor Pro',
'from' => '3.1.0',
'to' => '3.2.0',
)]
2021-04-18 19:01:43 [info] Elementor data updater process has been completed. [array (
'plugin' => 'Elementor Pro',
'from' => '3.2.0',
'to' => '3.2.1',
)]
2021-04-20 14:11:23 [info] Elementor data updater process has been queued. [array (
'plugin' => 'Elementor',
'from' => '3.1.4',
'to' => '3.2.0',
)]
2021-04-20 14:11:26 [info] elementor::elementor_updater Started
2021-04-20 14:11:26 [info] Elementor/Upgrades - _v_3_2_0_migrate_breakpoints_to_new_system Start
2021-04-20 14:11:27 [info] Elementor/Upgrades - _v_3_2_0_migrate_breakpoints_to_new_system Finished
2021-04-20 14:11:27 [info] Elementor data updater process has been completed. [array (
'plugin' => 'Elementor',
'from' => '3.1.4',
'to' => '3.2.0',
)]
2021-04-20 14:11:28 [info] Elementor data updater process has been queued. [array (
'plugin' => 'Elementor',
'from' => '3.1.4',
'to' => '3.2.0',
)]
2021-04-21 14:00:18 [info] elementor::elementor_updater Started
2021-04-21 14:00:18 [info] Elementor/Upgrades - _v_3_2_0_migrate_breakpoints_to_new_system Start
2021-04-21 14:00:18 [info] Elementor/Upgrades - _v_3_2_0_migrate_breakpoints_to_new_system Finished
2021-04-21 14:00:18 [info] Elementor data updater process has been completed. [array (
'plugin' => 'Elementor',
'from' => '3.2.0',
'to' => '3.2.1',
)]

PHP: showing 1 of 1PHP: 2021-04-20 14:19:23 [notice X 1][/home/sites/7a/3/3e1894e553/public_html/wp-content/plugins/elementor/core/common/modules/ajax/module.php::171] Undefined index: data [array (
'trace' => '
#0: Elementor\Core\Logger\Manager -> shutdown()
',
)]
```