[BUGFIX] Handle empty field array in FormEngine

There might be use cases, a record is requested
while the table and record type combination does
not contain any fields to render (empty showitem).

This happens, e.g. because a record used for
inline elements only does not define any showitem
by default but via corresponding overrideChildTca.
If such record is now requested without the parent
context, e.g. via link in the references table of
the element information view, a corresponding
message is displayed instead of a "undefined
array key" PHP warning.

Resolves: #105491
Resolves: #104963
Releases: main, 13.4, 12.4
Change-Id: I81ae0807c73af938c15af03248692bc33f2d82f0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86806
Tested-by: Benni Mack <[email protected]>
Tested-by: Oliver Bartsch <[email protected]>
Tested-by: core-ci <[email protected]>
Tested-by: Jochen Roth <[email protected]>
Reviewed-by: Benni Mack <[email protected]>
Reviewed-by: Jochen Roth <[email protected]>
Reviewed-by: Oliver Bartsch <[email protected]>

Configuration/TCA/tx_styleguide_type.php

@@ -33,6 +33,7 @@
                     ['label' => 'type 0', 'value' => '0'],
                     ['label' => 'Type with changed fields', 'value' => 'withChangedFields'],
                     ['label' => 'Type with columnsOverrides', 'value' => 'withColumnsOverrides'],
+                    ['label' => 'Type with no fields', 'value' => 'withoutFieldsToRender'],
                 ],
             ],
         ],
@@ -82,6 +83,9 @@
                 ],
             ],
         ],
+        'withoutFieldsToRender' => [
+            'showitem' => '',
+        ],
     ],
 
 ];