fix: pass default stamp space avatar address (#158)

It's usually it's set to space address but when creating new space
we don't have space ID yet. Before it was passed as undefined, which
worked (ignoring Vue warning), but now it fails as we try to format
address.

This PR replaces undefined with valid address.

apps/ui/src/components/FormProfile.vue

@@ -49,7 +49,7 @@ const definition = computed(() => {
         type: 'string',
         format: 'stamp',
         title: 'Avatar',
-        default: props.id
+        default: props.id || '0x2121212121212121212121212121212121212121212121212121212121212121'
       },
       name: {
         type: 'string',