[BUGFIX] Allow maxitem=1 in TCA treeSelect again

Introduced with ec7b2297 #75519, it was not possible to set maxitem=1 to
TCA treeSelect. This occures e.g. on setting the parent of a
sys_category.

The reason is the "selected" array which gets initialized with all
selected items. If there is no item selected, it is initialized with an
empty value and "selected.lenght" is 1 but should be 0. Selecting a new
value is impossible now.

The proposed path checks if items are selected or not on
initialization.

Change-Id: I0c8d21ad753dccb2d993febde404afc83a39b9b3
Resolves: #75947
Releases: 6.2
Reviewed-on: https://review.typo3.org/47937


Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>

typo3/sysext/backend/Resources/Public/JavaScript/tree.js

@@ -191,7 +191,7 @@ TYPO3.Components.Tree.EmptySelectionModel = new Ext.tree.DefaultSelectionModel({
 TYPO3.Components.Tree.TcaCheckChangeHandler = function(checkedNode, checked) {
 	var exclusiveKeys = this.tcaExclusiveKeys.split(','),
 		uid = '' + checkedNode.attributes.uid,
-		selected = Ext.fly('treeinput' + this.id).dom.value.split(',');
+		selected = Ext.fly('treeinput' + this.id).dom.value.length ? Ext.fly('treeinput' + this.id).dom.value.split(',') : [];
 
 	this.suspendEvents();