[BUGFIX] Provide default DS for pages_language_overlay (#399)

Prevents the following Error in Backend:

#1463826960: TCA misconfiguration in table "pages_language_overlay" field "tx_fed_page_flexform" config section: The field is configured as type="flex" and no "ds_pointerField" is defined and "ds" is not an array. Either configure a default data structure in ['ds']['default'] or add a "ds_pointerField" lookup mechanism that specifies the data structure (More information)

Configuration/TCA/Overrides/pages_language_overlay.php

@@ -13,13 +13,19 @@
             'label' => 'LLL:EXT:fluidpages/Resources/Private/Language/locallang.xlf:pages.tx_fed_page_flexform',
             'config' => [
                 'type' => 'flex',
+                'ds' => [
+                    'default' => '<T3DataStructure><ROOT><el></el></ROOT></T3DataStructure>'
+                ]
             ]
         ],
         'tx_fed_page_flexform_sub' => [
             'exclude' => 1,
             'label' => 'LLL:EXT:fluidpages/Resources/Private/Language/locallang.xlf:pages.tx_fed_page_flexform_sub',
             'config' => [
                 'type' => 'flex',
+                'ds' => [
+                    'default' => '<T3DataStructure><ROOT><el></el></ROOT></T3DataStructure>'
+                ]                
             ]
         ],
     ]);