MC-3723: Advanced properties saving when invalid value is specified

- fix issue with heading margin padding values on init

Author: anthoula

app/code/Magento/PageBuilder/view/adminhtml/ui_component/pagebuilder_heading_form.xml

@@ -136,7 +136,7 @@
         <field name="margins_and_padding">
             <argument name="data" xsi:type="array">
                 <item name="config" xsi:type="array">
-                    <item name="default" xsi:type="string">{"margin":{"top":"","right":"","bottom":"","left":""},"padding":{"top":"","right":"","bottom":"","left":""}}</item>
+                    <item name="default" xsi:type="null" />
                 </item>
             </argument>
         </field>