If no usage for a mailbox is known but status is fetched, fallback to 0 bytes

dashboard/src/views/EmailMailboxesView.vue

@@ -164,7 +164,7 @@ async function refresh() {
       if (!m.owner) m.owner = groups.value.find(g => g.id === m.ownerId) || null;
 
       m.ownerDisplayName = m.owner ? (m.owner.username || m.owner.name) : '';
-      m.usage = usage[m.fullName] || null;
+      m.usage = usage[m.fullName] || 0;
     });
 
     tmp = tmp.concat(result);
@@ -241,7 +241,7 @@ onMounted(async () => {
       <TableView :columns="columns" :model="filteredMailboxes" :busy="busy" :placeholder="$t(searchFilter ? 'email.incoming.mailboxes.noMatchesPlaceholder' : 'email.incoming.mailboxes.emptyPlaceholder')">
         <template #aliases="mailbox">{{ renderAliases(mailbox.aliases) }}</template>
         <template #usage="mailbox">
-          <span v-if="mailbox.usage">{{ prettyDecimalSize(mailbox.usage.diskSize)  }}</span>
+          <span v-if="mailbox.usage || mailbox.usage === 0">{{ prettyDecimalSize(mailbox.usage.diskSize)  }}</span>
           <span v-else>{{ $t('main.loadingPlaceholder') }} ...</span>
         </template>
         <template #quota="mailbox">