[BUGFIX] Do not render unused attribute in Checkbox element

Due to refactoring in FormEngine back in 2015, the Checkbox
Element renders " ... 1 ... " when a checkbox element is not
disabled.

To overcome this, the ternary operator is turned around,
returning an empty string when the element is not disabled.

Resolves: #87754
Releases: master, 9.5
Change-Id: Ic5fad1335fbdf1ed76523f598c20f66bfe195686
Reviewed-on: https://review.typo3.org/c/59778
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/backend/Classes/Form/Element/CheckboxElement.php

@@ -201,7 +201,7 @@ class="checkbox-input"
                     value="1"
                     data-formengine-input-name="' . htmlspecialchars($additionalInformation['itemFormElName']) . '"
                     ' . $checkboxParameters . '
-                    ' . (!$disabled ?: ' disabled="disabled"') . '
+                    ' . ($disabled ? ' disabled="disabled"' : '') . '
                     id="' . $checkboxId . '" />
                 <label class="checkbox-label" for="' . $checkboxId . '">
                     <span class="checkbox-label-icon">