fix: ensure description is always available in upload metadata form

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2307>

front/src/components/channels/UploadMetadataForm.vue

@@ -22,7 +22,8 @@ const props = withDefaults(defineProps<Props>(), {
 })
 
 const newValues = reactive<Values>({
-  ...(props.values ?? props.upload.import_metadata ?? { description: '' }) as Values
+  description: '',
+  ...(props.values ?? props.upload.import_metadata ?? {}) as Values
 })
 
 const isLoading = computed(() => !props.upload)