[BUGFIX] Allow to switch clipboard mode to copy without items

This change allows to change to "Copy Mode" in the clipboard
without having items on the clipboard.

However, switching to a different clipboard will have this
selection vanish again (before and after the change). Changing
this behavior is considered out of scope for this patch.

Resolves: #89726
Releases: master, 9.5
Change-Id: I83483c631915b390dfb0c9584c29f0f86a44d051
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62453
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Susanne Moog <look@susi.dev>

typo3/sysext/backend/Classes/Clipboard/Clipboard.php

@@ -183,7 +183,7 @@ public function setCmd($cmd)
         }
         // Set copy mode of the tab
         if (isset($cmd['setCopyMode'])) {
-            $this->clipData[$this->current]['mode'] = $this->isElements() ? ($cmd['setCopyMode'] ? 'copy' : '') : '';
+            $this->clipData[$this->current]['mode'] = $cmd['setCopyMode'] ? 'copy' : '';
             $this->changed = 1;
         }
     }