Fix property merge

Steps to reproduce:

1. deploy this contribution

   <component name="my.component.name">
     <require>org.nuxeo.ecm.core.CoreExtensions</require>
     <extension target="org.nuxeo.ecm.core.schema.TypeService"
         point="schema">
       <property schema="dublincore" name="created" secured="false" />
       <property schema="dublincore" name="modified" secured="false" />
       <property schema="dublincore" name="creator" secured="false" />
       <property schema="dublincore" name="contributors" secured="false" />
       <property schema="dublincore" name="lastContributor" secured="false" />
     </extension>
   </component>
   

2. start Nuxeo
3. observe the logs with this error

   ERROR [ComponentManager] Component service:org.nuxeo.ecm.core.schema.TypeService notification of application started failed: defaultObj
   java.lang.NullPointerException: defaultObj
   	at java.util.Objects.requireNonNull(Objects.java:246) ~[?:?]
   	at java.util.Objects.requireNonNullElse(Objects.java:301) ~[?:?]
   	at org.nuxeo.ecm.core.schema.PropertyDescriptor.merge(PropertyDescriptor.java:118) ~[nuxeo-core-schema-2021.8.6.jar:?]
   	at org.nuxeo.runtime.model.DescriptorRegistry.merge(DescriptorRegistry.java:92) ~[nuxeo-runtime-2021.2.52.jar:?]
   

It's a regression since the cleanup from NXP-29261