Bug: echo($addOn) condition is wrong in Magento/Backend/view/adminhtml/templates/widget/form/renderer/fieldset/element.phtml (and my fix)

Issues
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1

See the line:

https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Backend/view/adminhtml/templates/widget/form/renderer/fieldset/element.phtml#L41-L41

echo($addOn) ? condition is wrong.
Please note that the echo function always returns void: http://php.net/manual/function.echo.php#refsect1-function.echo-description

Also the $addOn has a logical value and there is no sense to echo it anyway:
https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Backend/view/adminhtml/templates/widget/form/renderer/fieldset/element.phtml#L15-L15

My fix:

Replace echo($addOn) ? to $addOn ?

https://github.com/magento/magento2/issues/2481