contribution to multi-tenant's 'configuration' extension point with empty 'membersGroupPermission' should not add ACE with empty permission to tenant's document at creation time

How to reproduce:

• contribute the following:

    <require>org.nuxeo.ecm.multi.tenant.contrib</require>
  
    <extension target="org.nuxeo.ecm.multi.tenant.MultiTenantService"
      point="configuration">
  
      <configuration>
        <tenantDocumentType>Domain</tenantDocumentType>
        <membersGroupPermission></membersGroupPermission>
        <enabledByDefault>true</enabledByDefault>
      </configuration>
  
    </extension>
  

• created domains will have the following ACLs:

  <acl name="local">
    <entry principal="EnerNOC" permission="Read" grant="true"/>
    <entry principal="Administrator" permission="Everything" grant="true"/>
    <entry principal="administrators" permission="Everything" grant="true"/>
    <entry principal="tenant-blabla017_tenantAdministrators" permission="Everything" grant="true"/>
    <entry principal="tenant-blabla017_tenantMembers" permission="" grant="true"/>
    <entry principal="Everyone" permission="Everything" grant="false"/></acl>